Because you'd e.g. be web browsing on a watch if it did have a web browser?
Besides, even with the "neutered apps" and only cursory use of them, the battery barely lasts a full day. Imagine with what you're suggesting...
I've had to charge my series 3 every night. I guess not having it paired with a phone somehow uses more battery power? Technically, I reset my wife's phone with my sim+apple id, paired, reset back to wife's backup and sim. So it has no real phone to pair with, though I guess maybe that causes it to use cellular a lot more than most.
As each series improves and Apple opens up new libraries so your watch becomes more and more independent of the phone, I believe we'll get more watch developers to up their game.
I was kinda surprised the first time I got an iMessage on my series 2 when my phone was in airplane mode!