Developers that aren't pushing microtransactions aren't worried about their players earning enough credits to buy a McLaren F1 in under 10 hours. How much of the design focus was on microtransactions is up for debate, but there's strong evidence that it's a pretty big part of their economy.
The low earning rate offline.
The incredibly low earning rate online.
The limited amount of races to use to earn in, meaning earning money is boring.
The high cost of tuning parts.
The high cost of the most desirable cars.
The limited time to purchase the most desirable cars.
The unknown availability of the most desirable cars.
The inability to sell cars. (Edit: Thanks
@Tornado )
The credit cap to encourage spending instead of saving.
The online only saves to ensure people can't go offline and mod their own games (and thereby avoid MTXs).
The really high MTX prices designed to extract the maximum profit from whales.