Take WoW, for example. Right now there are several million players logged in, busy beating the tar out of several million bad guys who are programmed to lose. They are playing, and paying good money, to have the illusion of challenge while winning the overwhelming majority of all encounters. (Why can't PVP replace this for most players? Because PVP implies someone is losing and the overwhelming majority of players want to decisively win the overwhelming majority of encounters. There is no way to accomplish those things without instructing a non-player to throw their matches.)
Let's say we replaced the AI with hired actors who played the monsters, and played poorly. Heck, we'll say the actors can even multitask 4 monsters at a time, all played to a similar level of poorly. At 60 monster-hours (15 man hours) per month per player, Blizzard would be in the red on accounts just from the actors salary unless their fully-loaded cost was under $1 a man-hour. Even if they got away with 40 cent wages for urban China and had minimal overhead this would slash their margins to bits.
So that's why we have AI: to do jobs humans won't do willingly (losing) at a price which is cheaper than the cheapest possible human implementation.