Bigbazz is right, as I see that even after I just purchased a car I am still allowed the option of going in and changing the oil, even though that's brand new. After racing or driving a car once, the option to fix the chassis or the engine opens up. However minute or massive the overhauls are, they all cost the same. I suspect that they do this to discourage players from doing teeny tiny repairs every race or so, because you wouldn't be doing that in real life.
Remember that all engines and all chassis don't wear equally: an endurance race would put a car through the wringer far faster than a single three-lap sprint. A car that has a redline of around 10000 RPM and can pull 2 lateral Gs regularly will stress its chassis and powerplant far more than a car with a redline of about 6000 and the cornering composure of a slice of French toast.
Also, remember that GT5 is a driving simulator that is meant to put the driver (you) at the center of the whole game's focus. In the real world, there's no light that comes on in your car next to the "CHASSIS NOW WIGGLY AS JELL-O" or "ENGINE IS MORE WORN THAN A *BLEEP*'S *BLEEP*." Rather, you have to feel those things. Case in point: one of my first purchases was a Renault Clio V6 Phase II. Only way I could get my hands on one was through the Used Car Dealership, and she already had more than 30,000 km under her belt. Despite my previous knowledge of the Clio being a mid-engined car with a nimble, responsive disposition (although there's also a prejudice against it that it understeers that I never got) the car felt like it had tires made of that plastic they make Barbie dolls with and the vehicle was wiggly and nervous, ready to step the tail out at almost any corner if I braked while turning. I immediately laid blame on the chassis losing rigidity, and it turned out I was right. As soon as I started driving that Clio, the effects of my new friend's mileage all came crashing down on me. I still won races, of course, but that was because it was all lower-level racing and I had changed the oil so power delivery was back to being smooth. I suspect I chose poorly because I ended up with a small French car that could absolutely blast down straightaways but went into a blind, flailing panic when the road got bendy and I had to lose speed quickly.
GT5 won't hold a player's hand because the game wants the player to learn through experience. As that player becomes more experienced simply from racing, earning licenses, all of that stuff, it would get pretty annoying if the game didn't trust the player's judgment.