Most people don't have $12,500 sitting around in the bank, or often even enough to rent a truck for the weekend. Some people work multiple jobs and can't even carve out a full day to move houses. Some people aren't in the best physical health (due to age or disability) and cannot actually move their own stuff.
You've described the typical SF tech worker and decided that everyone is like that. Might want to broaden your horizons a bit.
Regardless, you don't get the security deposit back until a few weeks after you've moved out. Never seen a truck rental or moving company who is all like "sure, you can pay us in a month". A bit of a chicken-and-egg problem, there.
