Of course, realistically, aliens wouldn't be humanoid at all. But it doesn't pay to approach these as black-and-white questions. Yes, it's a work of fiction, but it's one that's generally made at least something of an effort to justify its conceits. Roddenberry's original series proposal and bible stated that the Enterprise
would preferentially visit worlds whose gravity, atmosphere, climate, etc. were fairly close to Earth-normal, as a justification for why the "alien" characters they met were so humanlike and the environments they visited were so much like a studio backlot or a Los Angeles-area hillside or desert or park. It's about striking a balance between plausibility and necessity -- using human actors and Earthly locations for lack of an alternative, but trying to justify it scientifically to the extent that one could.
Why not 1.4g
? Because that's high enough that human characters would be noticeably affected by it and would be likely to remark on it, neither of which is the case in canon. What we see onscreen is that Vulcan gravity is not noticeably an issue for humans; therefore the logical conclusion is that it's fairly close to Earth gravity. Even Mandel's 1.251g
might be a bit steep.