And I agree, any normal player can afford every car under 2M with credits. However, quite a few people, maybe even the majority, are going to struggle to make enough credits to buy the 20M cars. And if they use their credits to buy the under 2M cars that means they are further reducing their chances of being able to buy the 20M because they have spent all their credits on the under 2M cars,
So presently you can't buy the 20M cars with real cash but their is an incentive for those who want the 20M cars to spend real cash on the under 2M cars thereby saving their credits to eventually buy the 20M cars.