When it comes to sports, it is always up to the players to do well. A good player could be good enough to be responsible for making a team good, average or bad. With most autosports such as Formula One and World Touring Car Championship, many drivers are restricted of their potential because of the limit of the car. A good driver can make a car work well under many conditions, but most of the time it won't be enough.
It stops a rookie driver from really showing how he/she can become a world-class driver, and it also means many big name sports have to hire drivers based on how much they cost rather than how much skill they can bring to the team.
Motorsporting is a sport that does not only test the driver's skill, but also how his team has made his ride. The driver is pitted against other drivers, and his team goes against the other teams. So the talent that one team has, of building the best cars, should not be taken away. Along the years, motosports has evolved to be a team game. If some team builds a great car, they also get recognition for their masterpeice.
Formula 1 and other high-level motorsports are team sports and every members of the team can make a difference. Development series on the other hand should (and most of them do) have spec cars to promote driver talent. Rookie Formula 1 drivers usually have been racing for over 10 years (mostly in spec series like Star Mazda, GP3, GP2), they have shown their talent. The good driver will eventually end up on the best team.