There was something of a collective and large-scale underestimation of how hard AI would be. Why that would be the case is interesting, especially since it persisted for some decades, across multiple fields filled with smart people. From probably the 1880s to, say, the 1970s, there seemed to be this widespread view that high-level AI was just around the corner, and mainly depended on some inevitable technical progress (faster computers and more memory, plus a bit of algorithms work).
There wasn't even really much debate, in either CS or philosophy or engineering, over whether computers would be able to do "routine" tasks like accurate object recognition, mathematics, playing chess, etc., in the near future. The biggest controversy was over whether computers could ever be "truly" intelligent and creative, e.g. whether computers would also replace Beethoven in addition to mathematicians, or whether they'd be forever limited to just being very capable automatons. Somehow everyone missed that even making them the "lesser" kind of intelligent, so they can walk around, recognize objects, translate languages, etc., would turn out to be pretty hard.