Why not hybrid? Have an onboard gas generator to recharge the batteries. As soon as the gas runs out, the drone has X min of battery flight time left to get back to solid ground. No need for mechanical complexity.
There's an MIT spinoff called Top Flight that was working on a hybrid quadcopter, IIRC. I think the main reason you don't see them in the hobbyist space is size floor (you need a pretty big machine to make the weight overhead of the motor, generator, and charge circuitry work out) and money.
Hell, you could have a small buffer ultra capacitor or battery providing the motors with electricity while a gas engine at constant RPM recharges it. The whole system would have to have a very high conversion efficiency though...
Gas engines could be coupled with one or several CVT transmissions?
I think you could just use the simplest form of CVT: a variable-diameter pulley under each rotor. You'd still pay a pretty big cost in terms of mechanical complexity, though, which makes the swashplate system more appealing as offers a lot of additional advantages in terms of thrust vectoring.
A few hobbyists have built gas quads with variable-pitch rotors, but the amount of mechanical complexity and hassle involved makes a similar nitro-powered single-rotor helicopter more appealing for most hobby applications (stunt flying etc).
As more novel designs that don't rely on precise motor speed control (for example, this one) come into play, I think gas motors will make a minor resurgence as people rediscover the energy density (and therefore flight time) they can achieve.
However, for most hobbyists, electric just makes more sense. It's usually quieter, works indoors, is generally a lot less of a pain in the ass to deal with (no need for starters, glow plugs, choke, adjusting mixture, engine maintenance, etc.), and is usually less heavily regulated.