Mostly physics. It's hard to do small jets, mostly because small things get too heat-stressed
I sorta watched a guy trying to build a hoverboard out of 50-kgf jets, it was crazy, hilarious and didn't go anywhere because flying a backpack of kerosene on four totatally unreliable jets ain't much fun in the end. They also cost about $5K each at the time.
Just yesterday, someone posted a link to a Veritasium video[0] explaining how a jet engine internal temps of 1500°C work when the components have a melting point of 1250°C. I couldn't imagine doing that at a small scale by hobbyists.
Besides, thrust control is shit even on their big brothers, on those, it was like throttle down - flameout, throttle up on the other two - flameout, oh crap, thank god we're doing tethered tests.
Gas dynamics on these scales are tricky too. Electric is the way to go for this.
I wonder if it's not a: "maintaining this kind of engine is a heck-of-a-lot of work and is why there are so many aviation regulations and the reason engine overhauls are forced and cost millions-of-dollars" kind of thing.
I sorta watched a guy trying to build a hoverboard out of 50-kgf jets, it was crazy, hilarious and didn't go anywhere because flying a backpack of kerosene on four totatally unreliable jets ain't much fun in the end. They also cost about $5K each at the time.