When I go through my match history (particularly on TT) it seems like it's actually more about them not spending the gold they do get.
Since bots don't intentionally last hit they have more passive gold generation, the problem tends to be things like them buying wards/sightstones they'll never use (SR) or no longer buying items at all once they have Doran's/boots/one finished item (possibly all maps, particularly TT).
The more noticable thing is that the older the champion is (by release date), the more likely they are to completely give up buying items at some point during the match. Newer champions (a relative term, given the lifeftime of the game so far) generally aren't made into bots but some of them like Lucian and often times Darius don't share this attitude of giving up after three purchases.