Simple, because they don't spend a lot of money getting the hottest driver to run races for them?....

Don't get me wrong, I find it ok that Manufacturers do, but on a WC equipment between manufacturers is very equal, it's the drivers that makes the difference...

I'm an 8th scale races and some years ago Jilles Groskamp also started running these cars for Serpent in the beginning.
At that time a friend of mine, capable of running 15.0 laps at out track, gave his car to Jilles, because he wanted to attend a clubrace to get the feeling of these cars.
After 30 minutes of driving, with the same car, Jilles drove 14.3 laptimes with it!
So 0.7 seconds faster, and not because he was running better equipment!....
But just because he has the skills.....