You might as well ask why the designers "fear" what happens if we let elves have human lifespans, or "fear" what happens if we let constructs reproduce biologically.I think it is more so great fear of what happens if we let the undead not be evil but I fail to see what that great fear is?
It's an aesthetic decision, based partly on the mythological roots of undead, and partly on the designers' personal preference. If you don't like that aesthetic, you're free to change it in your campaign. Personally, I think it gives animated undead much more resonance than making them smelly robots.