The game is obviously designed with the ability to allow you to unlock things at certain times, and then while they're unlocked you must make the money to buy them. This system of fresh upgrades and money-making requirements keep you doing missions which keep you actually playing the game.
What keeps you playing the game is that it's fun, combined with the additional content that will continue to be released over time.
Also your rank provides your progression if you need that kind of motivation, - car mods require rank and race wins to unlock, as do weapons and their mods.
Not having enough cash just means instead of experiencing every part of the game, you tend to focus on high payout jobs or grinding.
The economy was more reasonable at release, but they've kept smashing it down patch after patch. The idea is, when you want something, you'll go to the game store and buy a GTA Money Pack.
The people who glitched and run around "I'm bored" just shouldn't be playing the game.
Do not think for a second that everybody does this as soon as they have a few million.