I mean I think it's nearly irrefutable that many, many GMs out there fudge, deploy quantum ogres, and/or "allow" things while then taking pains to ensure that the thing in question is functionally impossible despite being "allowed". If a person considers those things cheating, and I absolutely do, then GMs are literally advised to cheat, sometimes by a variety of professional and otherwise well-respected sources. That, alone, indicates there's way, way more opportunities, and more incentives, for GMs to "cheat", if one agrees these things count as cheating. I know a number of people also think it literally isn't possible for GMs to cheat, and thus they engage in such behaviors without restraint, which is rather different from players. Even a dirty rotten constant cheater recognizes that what they're doing is cheating most of the time. They just don't think it's a big deal or whatever. GMs, again from my perspective, frequently deny that their actions even are cheating in the first place.