I would say it's because the alternative to photorealism is usually just crap. Key word "usually." If you have photorealism, at least you have that. Something that can be appreciated objectively.

If renders aren't going to be photorealistic then I want something at least as good, and I think that's a tall order a lot of the time.

edit
Let me hasten to clarify that I'm not saying non-photorealistic renders tend to be crap. I'm saying you don't get anything by sacrificing photorealism, most of the time. So if you can have photorealism, it's usually a good idea.

It's not the tool used, it's the tool using it