In making the cars harder to obtain it imparts a degree of rarity or value. It's not just the cost of the cars, it's the invitation only cars, the cycle of availability in the Used car and Legendary car dealerships, and the barriers to 'building' your own 'unicorn' car (i.e. engine swaps and S parts). Ultimately what the developers are asking of you (and offering you) is a long term time investment, and I think it's fair to assume their incentive is that in some way they benefit from prolonged interaction with the game, and possibly because they think giving you that emotional investment in the cars makes the game better. Whether or not that's what you get out of it likely depends on your attitude to the game, but if you've put a lot of time into something you're more likely to value it when you achieve it*.
In principle, this is one of the cornerstones of the franchise (race, earn, buy), if you don't like the concept of that, GT isn't the game for you - that said, the time investment required versus the things to do to occupy that time seem much less favourable this time around, to the extent where it seems obvious that a sensible way of prolonging players interaction with the game would just be to give them more to do, or tweak a couple of features to make them more accessible.
* A paradoxical example of this, for me personally, is liveries. I've put A LOT of time into making liveries, and am proud of them - a big part of that is because they are hard to do. Now, most people would probably prefer PD made it so you could just upload PNG files of any size, rather than being restricted to 15kb SVG's, and it's tough to argue that wouldn't make the game better for many people - but at the same time, it would remove a sense of accomplishment and achievement that I value.