Exactly what I have been saying; what is the rush?
It's not about a rush. It's about the ease with which you lose the money you gain. Dying in Free Roam can cost you as much as most missions <level 20 earn you. And with the ease that people can gank/kill you, it can amount to a very serious loss of monetary progression.
Take into account that you can't own more than one car until you buy a Garage (cheapest of which is $25K) and you're severely limited.
I can tell you right now that if there weren't missions that made >$3k, people would be doing nothing but Non-lethal Races and avoiding Free Roam to prevent loss of progress. There are many ways they could have fixed this though.
1. Make Hospital Bills come out of the cash you carry on-hand and limit deposits to physical ATMs. This way, if you can make it to an ATM to deposit, the money deposited is safe. I'm willing to bet people would rather go into a cash-on-hand debt from death rather than losing deposited money.
2. Balance mission payouts by difficulty. Short, easy drug deal raids should be low amounts and long arduous high-chance-of-death missions should pay considerably more. People are much less likely to farm high risk missions that take 10+ minutes per run with a full 4 man squad. Risk vs. Reward would still make them valuable however.
3. Allow players to have a Starter Apartment/Motel Room with 1 parking space, allowing for a single vehicle to be safely stored and 1 vehicle to be driven in the world simultaneously. The two biggest reasons at this point to own a home is for vehicle storage and safe havens. Giving players a really basic starting point for that would help alleviate the desire of players to farm missions.
None of those would trivialize the time it would need to take to level or gain money and that's just off the top of my head.