Because seriously, what's the point of lovingly souping up your cars and bikes for hundreds of thousands of bucks, and then not race 'em for reals? Yes - it's unfair if you haven't got a sweet ride of your own, but does it really matter if it isn't? As far as I'm concerned, it's what the whole *persistent world* angle to gaming is all about. Otherwise, why even bother with a persistent world and character progression? Just play any damn non-persistent game, which are inherently designed to be more even-handed and fair.
Racing without custom vehicles and catchup on is like what Blizzard has done to World of Warcraft when it introduced resilience (mitigates PvP damage/control effects). They broke the game's cohesive persistence in two - by establishing two completely seperate paths of itemization - into PvE and PvP progression. That's when the game's persistence began to become meaningless. So you are one of the legendary heroes that vanquished the world's greatest evil first? It doesn't matter in the world at large. Some intermediate skill-level goon in resilience gear will akin to an untouchable god to you. You are the greatest Gladiator that ever lived? Well - that monster there doesn't care about that. You are powerless. Before resilience, I was a Warcraft player. It was one game, one world, one life, one love. After I was either a PvE or PvP player, or both - but never whole again.
So for the love of god, embrace the tuning culture and tune-up rides to race them, because that's how it's supposed to be. If you are a racer, you want your car to be the best. You deserve an advantage over the racing peasants who race stock cars. Racing culture is more than just showing up at the starting line. It's striving to buy a sweet ride, and tune it up to the gills with power, and adding frills to make it your own. Ontop of that - whitewashing races with the catchup setting is the worst thing ever. So you get ahead of the pack, don't fuck up, and yet whatever you do, you'll get slower and slower, and if worst comes to worse, the timing works out just right that you don't have a chance in hell to win anymore when they finally catch up, breezing by with slipstream speed just before the finish line while you're stuck with a crippled gaspedal from catchup.
btw. decent matchmaking would go so fucking far to accommodate for both, but damn if it isn't shit. I'm sitting in a lobby right now. Already waiting a half an hour for somebody, anybody to matchmake into it. If matchmaking was better, I'd have lobbies full of serious racers with souped up rides all day long. Grrr! I mean how do you manage to matchmake millions of people worldwide and end up with empty lobbies abound? By being really fucking bad at it. That's how!