Same reason you play LFS when you could just go and play NFS...
Personal preference. Sheesh. And trust me, the same talent used to create pixel cars could not go into creating higher res versions. Pixel tuning and virtual tuning are COMPLETELY different - I know, having done both. When i made the change from virtual tuning to pixel tuning i had to learn completely new skills. I'd never even used the pencil tool when doing virtual tuning. When you're doing a tune on a large size, you can get away with making mistakes because they just get hidden. But on a pixel car, every single piece of work you create and be seen, and mistakes easily spotted. If anything I find pixel tuning more difficult than virtual tuning.
Pixel cars are just as satisfying to complete as a full size virtual tune. It's about attention to detail, ir originated from japan, and we all know they have excellent attention to detail.