I can't say for sure about the habitual player-only type, but I might share some of the problems. For me, my reluctance to resume after burn-out was, and remains, centred about the problem of excessive GM authority in anything adjudicated. It's about not being able to sustain the necessary illusion of player free will from my PoV, while keeping the amount of GM prep needed to a manageable level. For a fully live quasi-simulation, a world that “runs itself”, in which the players can really do what they want, the workload is enormous (at least compared with the amount of time and energy I feel I can devote to it these days). That's before one faces the possibility that every player takes off on a separate agenda (as helped crash my V:tM chronicle). For anything constrained, I have to guide the players, and this feels wrong.
On top of that, choosing a system, choosing how to customise it, can all lead to analysis paralysis even before getting to the crunch point of selecting a setting, and trying to set it in motion. And then you have to worry about whether your players will be happy to consume what it is you have to offer, whether they will propose characters that you can handle, and that won't do violence to what you wanted to run.
It was all so much simpler when a satisfying evening's gaming could be put together by selecting opponents for two fight scenes and a skeleton of narration to join them up. Things started to go downhill when gaming evenings had to stop at half ten rather than one a.m., and violence had to be suppressed after the 9 o'clock watershed and/or speeded up by abstraction, and other things had to be introduced to fill the space. Nearly as good are the few very best pre-packaged adventures where the GM can just play the hand he's been given without needing to rewrite it to eliminate crass bits, and allow the time for the game system to produce results, rather than have to also make them up.