"Can't you just take care of all this usability stuff by following some simple rules?"
Yes and No. If developers could make usable apps from the confines of their cube, they would.
Usability is a measure of prevention, if done right, you are never going to see where it paid off (unless you are building a better mousetrap).
“ I think saving small fractions of a second by optimal button placement is probably a good illustration of the real but limited impact that traditional psychological theory can have if diligently applied.” (Carroll, p. 65)
"Why Can't We Do This All In-House?" (cont'd)
"Aren't you smart enough to figure out what the user needs on your own?"
“ Personal observation of one’s own system is one of the worst ways to assess usability." (Carroll, p.69)
Turn that question around. If you can't prognosticate that, what makes you think a developer can?
"I know exactly what the end users need, it's ..."
Ok, maybe you do maybe you don't. Let's not guess, lets find out for sure. That's where field studies and user interviews come in.