There exists a square-cube law (or something similar) so that computation becomes less and less efficient or precise or engineerable as the size of the computer or the data it processes increases, so that a hard takeoff is impossible or takes very long such that growth isn't perceived as "explosive" growth. Thus, if and when strong AI is developed, it doesn't go FOOM, and things change slowly enough that humans don't notice anything.

One possible explanation, why we as humans might be incapable of creating Strong AI without outside help: * Constructing Human Level AI requires sufficiently advanced tools. * Constructing sufficiently advanced tools requires sufficiently advanced understanding. * Human brain has "hardware limitations" that prevent it from achieving sufficiently advanced understanding. * Computers are free of such limitations, but if we want program them to be used as sufficiently advanced tools we still need the understanding in the first place.