Have your teacher, or anyone else told you the “reason” for “2 eyes”? It’s not part of the rule, but a combination of the rules allows a group with certain properties to be kept without being captured if opts for a pass.
The 3 rules involved are
- one side can only play one move in a turn
- The capture rule checks the liberties of the opponent’s group first, and removes them, before checking the liberties of your own group (hence a group with just one single intersection left connection to it, even if it is within the group, can still be captured)
- a player can opt to pass instead of playing a stone on the board
If any of these 3 rules are changed, then the property allowing a group to stay on the board will change. Like the new game combined cards and Go I posted, allow more than one stone to be played on the board according to the cards drawn, meaning, more than one stone can be placed in one turn, hence you will need to be careful to check if your opponent actually got a card that has the exact shape of stones fit into your eye space.
And for some Atari Go (especially first capture wins), the pass move is not an option, so if no players ever capture a stone, when both sides have one group and 2 single-space eyes left, the one who plays next will be forced to fill one od the eye, and allow the opponent to capture the whole group (thus preventing draw).
(And changing the capturing rule, obviously will change what kinds of groups can stay alive on the board). If you understand the reason why these properties allow a group to stay alive, then you will also understand why seki is just another form of this property without the need to “define” any group “alive or dead” (or even define what is an “eye”).