For sure miniature gensets exist! They're often used on gas RC / autonomous airplanes to generate supplemental electricity. Here's one: https://www.youtube.com/watch?v=1yB49G756OI
I think the answer is that the miniaturization reduces efficiency, adds a ton of tiny, expensive parts/maintenance, and doesn't generate a lot of power compared to what you can store in a battery. So while the idea is cool, it's really only applicable in a few specialized use-cases like long-duration drone flight.
You've got to also compare it to other available power generation schemes -- A single 250W solar panel is far cheaper, simpler, and eco-friendly.
I think the answer is that the miniaturization reduces efficiency, adds a ton of tiny, expensive parts/maintenance, and doesn't generate a lot of power compared to what you can store in a battery. So while the idea is cool, it's really only applicable in a few specialized use-cases like long-duration drone flight.
You've got to also compare it to other available power generation schemes -- A single 250W solar panel is far cheaper, simpler, and eco-friendly.