Because it's better to have choices. You can personalize your car. Don't like the stock look? Customize it to look as badass as you want. It's all about the freedom of expression. Not everyone is satisfied zipping around in a car that looks the same as many others. Make it as unique as you are.
Well, the big deal about newer NFS' not having customization is the fact that it's hard to make something unique to you. What's the point of going to an Italian restaurant if the only thing they have is cheese pizza? Some people like cheese pizza, but it's nice to change things up abit. Same with cars, IRL and in NFS. It's all about personalization and variety. Even if all you do is replace the rims and put a decal across the windshield, it's still better than having a 100% stock car with maybe 2 or 3 different colors to choose from.
The bad thing about customization in NFS is that there's either too many options or too little options to choose from. In The Run, as an example, the cars had different style packs that you could apply, and that was alright, but it didn't change the performance of the cars that much, and you still couldn't personalize them that well. NFSU2 on the other hand, I guess you could say it had too much customization. Double roof scoops, the new bumpers ( specifically the level 3 ones ), hydraulics, spinners, and the mirrors, that was going too far lol. Spinners are cool sometimes, and hydros go good with lowriders and SUV's, but running hydros, double roof scoops, and Triton bumpers all around on a pink and green 206 with the Burger King vinyl on the side is plain ridiculous IMO. And it's a shame when people can do that to cars like Skylines and AE86's. Now MW 2005 and NFSU had the perfect amount of options for the cars that were in both of those games.