Briefly, it’s for ranking system stability.
If lots of people join, start games, and leave by timeout, it is said that the effect will be an upward trend on all ranks on the server, which is not desireable. A lot of care is taken to make sure that the overall ranking stays stable (this also is why new people are 13k with high uncertainty).
If you are interested in more details, and to see the various arguments on the table for and against, you could start here: Escaping by Timeout