Internal hubs now can have just as much range of ratios as derailleur's.

They also have the benefit they can change gear while stationary (very handy for traffic lights etc)

Historical (ie. 1960's) internal hubs only had 3 gears, so what you say might have led to market dominance.

Only the expensive ones have the range and you can buy a complete entry level bike for the price of a decent one. They are still heavy and have more than noticeable drag.