I suppose its a mute question with Secret Wars on the horizon, but we know the original Guardians from 3k are from an alternate universe the diverged from 616. I think it was because Vance Astro meet himself in the past or the Martian Invasion.

The question is, why when the Guardians time travel do they always end up on current Earth 616. I understand its for story purposes but has Marvel explained this?

@earthsmightiest: I think its supposed to be the future & past of 616.

The past stays (mostly) the same but future is mutable, think Back to the future(s)-movie here.