Size, noise, maintenance, predictability, simplicity, cost. Haven't seen a power/weight trade study of batteries vs engine+generator+fuel, but it may be closer than you think. There's a lot involved in a ICE power plant, and even if it did win, it'd have to be a pretty big machine compared to your average multirotor due to minimum scale on gasoline engine components.

There might be a market for large endurance multirotors, but apparently it's not obviously attractive.