Because they shouldn't do that in the first place. What is the point of making an event open to a whole continent of cars, only for GT5 to consistently choose the same small group of cars for you to race against, and not adapt to what car the player chooses? That is beyond stupid, and makes no sense whatsoever.
Has to do with how they divided the events. There are separate events for Lamborghini and Ferrari. It seems they were thinking of non-super cars when they made the European Championship, so there aren't any in there. It doesn't scale because those aren't the parameters of the race, even though the name implies otherwise. It's kind of like complaining that the game doesn't scale to your Ferrari on the Sunday Drive race. It's supposed to be low level, even though a Sunday Drive could probably involve millionaires and their Zondas, that's not what the entry level races are going to show you.
Put differently, these races are an excuse for you to buy or actually use that standard Euro car that you won in an earlier race but probably forgot about and is rusting in your garage.