And yet making those first two things so they don't look like they were designed by a blind simian requires extra time and know-how.
Actually, careful examination of modern titles indicate that it's a question of demand.
Average gamer can't be said to be as dumb as a brick only because such statement would be insulting to masonry.
This basically means that trying to introduce any demanding system such as good AI or stealth that wouldn't be based on exploiting dumb vulnerabilities is a bad move when you're trying to sell your game.
Take DX:HR with its patrols actively avoiding peeking behind obvious cover, Dishonored where the AI never looks up, or Skyrim with actually decent AI that has been set to forget it was searching for you before it got halfway there.
Those are purposeful design choices to make games playable by utter morons. Those same devs could probably make much better stealth expending the same amount of time and effort if not less.