Every company is likely to have slightly different reasons to want to build their own 3D models of cars. One reason they will all likely have in common at some level of priority is that the design of a car is something covered by intellectual property rights in most places, and that means that the game developer must obtain the legal rights to use the car designs in a game.
That is potentially a very involved legal process (getting in touch with the right people at the car company, negotiating licensing deals, making sure the deal covers all the important rights each party would need to obtain, et cetera). This tends to be extremely expensive both to pursue, and then ultimately to execute on (to pay for the license).
It's thus not likely to be considered extremely important to have real life cars in a game, since it's usually not something that constitutes a "killer feature" and the cost to get there doesn't usually balance out the payoff.
Other reasons a company may want to simply build their own models (including reasons why the deal might fall through) could be:
the game company might have the money on hand to pay for the aforementioned legal processing, but they might not have the time to wait for it to happen
no deal may be available that satisfies the game company's needs; for example, the car company may only be willing to offer a license for three years, after which point the developer must cease use of the cars, thus potentially forcing the developer to stop selling the game at the end of the license agreement (for reference, consider how Alan Wake was recently pulled from Steam due to expiration of some music rights)
getting a licensing deal with one car manufacturer may prevent (financially or legally) getting a deal with another manufacturer
the 3D models car manufacturers have on-hand may only be extremely high-resolution CAD models or some such, suitable for their needs, but quite unsuitable for the needs of a high-performance game; the game company will thus need to employ artists to streamline those models for in-game use, and they may be hamstrung by legal clauses in the contract regarding how much they can modify the art
the game's setting may not lend itself to having real-world cars in it anyway
Note that in the case of your friend's company, which is doing outsourcing work for other companies, it may be even harder or more expensive to obtain rights to use the car models (to build appropriately game-ready models). That's because your friend's company probably needs to obtain both the rights to use the car designs and to redistribute those designs and licensing to other studios. That's a powerful right that many car companies, indeed may companies in general, are unwilling to part with cheaply or at all.