There are a lot of complaints these days about the grind needed to afford cars
They drastically nerfed the payouts recently too, and they were already pretty low, so a lot of people are pissed. The game has microtransactions - the fastest way to get money is to use your credit card. People speculate that nerfing the payouts was a scheme to incite us to buy money etc.
That and there are cars that you can only buy for a limited time - thus "forcing" you to buy money in order to not miss your chance to get the car
You can't sell cars either
So tl;dr - it takes forever to make money, enticing you to use a credit card to cope with the struggle. That's what people are saying anyway
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Time taken to buy just one example of the most expensive vehicles in each game:
Gran Turismo 1 – 1 hour (8 races)
Gran Turismo 2 – 1.5 hours (14 races)
Gran Turismo 4 – 2.7 hours (32 races)
Gran Turismo 6 – 5.8 hours (90 races)
Gran Turismo 3 – 7.5 hours (5 races)
Gran Turismo Sport – 9.9 hours (84 races)
Gran Turismo 7 (v1.06) – 10.9 hours (205 races)
Gran Turismo 7 (v1.07) – 17.8 hours (333 races)
Gran Turismo 5 – 23.3 hours (315 races)
Again though, this doesn’t reflect the fact that GT5 and GT6 had daily login bonus boosts and quicker ways of earning credits through seasonal events during their active life. That would promote both games up the list, with GT5 eclipsing Sport and 7 during its heyday
As for me I only started recently and it's been fun so far. I'm currently trying to gold all license tests. I have S-7 and S-10 left to do, I haven't tried them yet. They're infamous for being the hardest so I kept them for the end