Because a restaurant is an experience that you can't easily recreate by yourself (usually). Someone carrying your bags is simply brute labor and in many cases isn't saving you any real hassle.

I'm also totally fine with restaurant tipping (and rather generous with it), but don't like tipping for bellhops/skycaps/etc. either, and will therefore avoid their services unless I really need them (i.e. if I have more bags than I can reasonably carry on my own). Of course, if I do use them, I'll tip appropriately, but I prefer to simply avoid them instead.