honestly, I have a hard time figuring out this very same thing about most fmls

That’s what I can’t figure out also. If I didn’t have to have a room mate, I don’t have one.

You know some people actually enjoy living with other people. That's not because you can do something that you have to do it.

Considering the OP seems to be female, and the flatmate is an entitled d*ckwad, there might be more to it than just "enjoys living with people". But I'm probably reading too much into it.

It's common practice: lots of rich people actually live very frugal live, it's part of the reason why they're rich. They won't spend money on things they don't need to and find every way they can to save money.