If AI responded/acted based on a predefined set of patterns that could be recognizable, the player would automatically feel it (pattern matching) and makes the NPC far less interesting.
Beyond reinforcing our tendency to project, as you say, a personal history on random behavior, it also highlights what a few other people have commented: that in many non-cooperative situations a committed random strategy is extremely effective, and perhaps more effective than a biased, seemingly "rational" strategy. (For another example, I believe Henrich's "The Secret Of Our Success" discusses the possible adaptive benefits of divination as a generator for random strategies among early societies.)
A lot of the best ml right now is effectively about making better conditional probability distributions. You always get random output, but skewed according to the circumstances, and sharp according to confidence in the result.