That is simply not true. You need to pay more attention to what is on your screen.
Example: Clubman 500 most cars will be some where around 500PP if you are an average driver then you would want to use some where around a 500pp car hence the name 500. If you look at the cars that show up some will/may be a bit over 500 but the limit of the race is actually 600. This allows poor drivers to have a chance as well.
Those of us who want an interesting race will use something less than 500.
There are a few races where you will run into 1 or 2 group C or lmp cars along with possibly some super cars and/or GT cars. Those races allow you to run any of those cars yourself as well.
As for the AI letting you win no matter what you drive that also is simply not true. There are lots of cars that you will not be able to win no matter how well you drive them. The AI will go slower if you are driving a slower car but they will not go slow enough for you to win. You will appear to be close, then you add some power and try again, faster this time but no closer until you finally get enough power to actually beat them then you will win and they will not go any faster than that so if you use and even faster car you will just beat them worse.
This of course assumes that you drive well. The AI will hold up in cases where you have spun out or crashed and if you are using a fast car you can still over take and win. If on the other hand you are using a car that will make a race of it then you are likely to loose if you spin out or crash.
For many of us it is much more fun running in a race they we know we will have to be perfect to win than to drive an overpowered car and use it like a plow to blast through the AI for an easy and meaningless win.
But with this Stupid AI slowdown this won't really work.
Because if you go out of a race to take a slower car and go back you get different opponents and it is a different race.
If the AI would stay at their normal speed and you could see what cars you are up against you could adjust difficult with your car.
But you never know which opponents you get and how much they will slow down.
The result is you have to do a race 10 times until you get it right once, and this is just awful.
Best examples are the races where that stupid FFX shows up, if you are against him you need something fast, if he's not in something 5sec slower will do.
Or the 600pp touring car champ., if the AI McLaren F1 GTR is in it is not easy to win with a stock R8 LMS, if the F1 is not in it is a walk in the park.
The AI slowdown is really bad, the opponent selection makes it much worse and the short (chase the rabbit) races are the final nail in the coffin.
The basic idea of "adaptive" AI is not so bad, but they got the balance totally wrong.
The AI are to slow anyway and the additional slowdown is just way to much.
It should be balanced in a way that a decent driver can win if he uses the same car (stock) on same tires as the fastest AI car (whithout it slowing down last half of the race).
As it is now it is horrible for decent drivers because you know that in reality you could never win with the sh..box you have to use to get a challenge.