Fine, since this is phrased as a serious honest question and it is a free day, I’ll give you some answers/examples:
a) Swap the flags - #65 by JethOrensin as you may have noticed my country failed for 35 years to even record whose house is where, during peacetime. For example, my uncle had a one-story house and a one-story storehouse. I took the paperwork to the land registry myself and everything was correct. Contracts, papers, satelite maps, coordinates, the lot. Result? According to the land registry now my uncle has NO house and two-story storehouse.
So, locating whose house is where, how big it was, how many stories it was, who owned its floor/story/apartment is an extremely difficult and arduous process. If you think this is an easy process, please send an email to the “Greek ministry of land”, they are in dire need of help.
Now add to this the fact that there is no peace in the region, those are very densely populated areas, a lot of buildings might have been torn down, a lot of the former residents might me long gone or dead and you have an even more complex issue at hand.
I had already written all that but you ignored them.
b) A lot of the occupied buildings are not even there anymore. Along with the people, corporations and building conglomerates moved in as well. This means that building blocks and small houses were torn down to become building blocks and malls and larger high-rise residential buildings. Let’s say that you lived in a small old house that is no longer there. Where are they going to return you to? To a corner of a mall or an office complex?
c) In no agreement that was ever made in the history of our species, has there been a deal where the side/person holding most of the power agreed to actually lose more things by signing a deal. Even the most incompetent leaders in history understood that “when you are strong, you get more when you close a deal” and that’s the main basis of diplomacy, ever since the times that “diplomacy” was conducted with clubs and stones outside caves. Israel is the de facto stronger part at this moment in history, ergo there is no way they would ever accept to 1) get all those people out, 2) pay for the reparations of the people they displaced and then 3) pay reparations for their own people that they had sent to live there with promises of a better life and 4) then pay even more to actually relocate them somewhere of equal value.
Even one of those four things would have been a dealbreaker for the power-holder of a deal. You want all four of them to happen and you call that “simple and easy”. 
d) The exact same problem still exists with Northern Cyprus. Things like this have happened before. Reading some history is always useful. As you will find out, no commonly accepted solution has been found yet and it has been more than 50 years now. Even if we discount the changes and urban development of the past 50 years and we somehow magically could return all those people and those areas back to the way they were 50 years ago, most of the people that were evicted are probably too old or even dead. Most of their children were either too young or not even born there. And where would all the people that moved in and have lived there for all this time go now? It is a tricky and complex problem. Which is why it is un(re)solved.
Do you want me to continue or is is now clear that this is NOT a “simple matter”?
Here is a very simple example:
Let’s say we are in a film noir and you go in and rob a bank. You get into the safe with some daring-do and you get all the gold from the personal mini-safes. While trying to escape with all the gold, the police apprehends you and they get the gold back.
You put all the gold in your bag without a care of which container/safe you took it from and without any documentation. You didn’t care because now all the gold was yours. However the police needs to 1) document the gold, 2) take stock of the list of people that were clients of the bank, 3) reference and cross-reference the contents of the sack compared with what the people declared that they had into their personal safes (the banks used to keep no record of personal safe contents - that’s why they were personal) and the process will take a lot of time because you can be good money that once a robbery has occured people are going to inflate the contents of their safes (only they knew what was in there, so by declaring more, they might get more if some other safe-holder is missing or doesn’t declare their own contents - these are real-life problems, people lie to the police or to insurance in similar cases all the time).
Unsurprisingly, putting things in a bag is easy. Giving them back is much harder.
Now do that with houses and acres and acres of land.
Especially in a place full of conflict and mostly devoid of laws/lawkeeping, no land-registries and unknown amount of deeds.
That final part is important.
Somehow you seem to assume that all those people even had deeds (newsflash: probably not) and that fleeing people are like “hey let’s run for it, but first let’s grab our bags with the deeds and the topographic diagrams of the house!!!” … Who does that? Not even a small fraction of people, is the answer.
So, let’s say you had a house of 80square meters and you know that your neighbour that had a similar house is dead. Now they tell you that the houses are gone, but you have a chance to get your land back and a new house. What are the chances that you declare that you had 160 square meters? 
Now put the other neighbours into the mix that they know that middle neighbour is dead and they all over-estimate their plot? Who will get those 80 square meters? And thus the squabble begins.
Maybe you have been blessed in life and never had to see nor hear about such occurances and you might think that such things are rare and that people will honestly get back into their places without any arguments or disagreements, but in rural Greece those are common-place and I can fill books with them. Actually a great deal of the problems with our land registry is that exact issue. Honest mistakes and the incompetency of the clerks aside, a lot of people found a chance to grab land from their neighbours and took a swing at it. They reckon that they have nothing to lose.
If their trick works? They get more land!
If it does not work? They get the land that they really have now and they go “sorry, my bad, oooops 
”
So what do they have to lose? Getting caught costs them nothing.
So, no risk and a lot of potential gain.
And thus their “real nature” as human-form vultures is revealed and that begun a country-wide land registering fight that has lasted 35 years and will last 35 years more (if we are lucky).
So, do you want more? Have you had enough? Do you finally see the complexity? Are you even going to read any of that or did you give up half-way? 