Correct me if I am wrong, but IR missiles have a relative short time to acquire target as their seeker needed to be cooled so you need a man to control it to turn it on at the right time.
That is true for older missiles with one-time-use chemical cooling cartridges or batteries, and old IR seeker tech that needed very low temperatures.
Now there is technology which can operate with lower differential temperatures, and can operate continuously with very low power input and without the need for huge cooling capacity.