The question was - I don't understand why people keep money in their old 401k accounts.

The answer is - there are situations where it is favorable to do so. It may or may not be favorable for you, and it may or may not be favorable for the median person. But these situations exist. And everyone can check for themselves and make the best decision insted of arguing over a blanket "X is better" or "Y doesn't make sense".

Sure, I agree with that general sentiment, but I think there’s probably a ~90% chance that you should rollover somewhere else.