If I'm able to find hundreds and hundreds of stimpaks, my enemies should too. Sometimes they do use it, but it doesn't seem to happen often. I remember in earlier games, they heal themselves a lot more often... Or am I thinking of the Elder Scrolls games?
But of course, this is a single-player game, where AI enemies *have* to act dumber than me in order to have a playable game. I was playing Elder Scrolls Online, where I could beat NPCs in seconds; but when I fought an online opponent with similar levels and skills, the fight lasted *15 minutes*, lol. In a SP game you can't have 15-minute fights all the time, lol.