I've always found that tuning in Forza makes cars feel more similar compared to driving them all at stock.
But there are plenty of reasons to upgrade (be it preference, for a specific race goal, for online, etc...). But my point was that if the game doesn't force you to upgrade then you can choose, which is great for everyone.
In FH4/FH5 you can upgrade cars to be whatever PI you like, and the AI will match you, however except street racing the AI will stick to the same basic class as you so you won't get Focus against supercars in most races.
Driving a truck in C class or in S class feels very different and the AI changes wildly too. The ability to do this is just really fun for me. As compared to FM7 where that truck could only race the AI at a specific homologation PI. That was boring in comparison.