[SPOILER]
I was thinking the exact same thing. I mean I get why AOS ignored the snap and decided to tell their own story (WHICH WAS AMAZING BTW) but it kinda bugged me when they didn't clear things up. One thing is for sure. Seasons 1-5 is CLEARLY part of the MAIN MCU timeline. Season 6 ignores the snap (for obvious reasons) so not sure about that and the final moments of season 7 is set as the continuation of season 6 so not sure about that either.
They could be in an alternate timeline which diverged JUST before the snap happened and the snappening did not happen in this timeline. That makes sense since the whole season 6 is kinda light-hearted and DEFINITELY don't feel like half the population of the entire universe just died. This would also explain why they didn't time travel to stop Thanos from snapping since the snapping never happens in that timeline. (Also I have a whole theory based on the divergence of timelines)
OR season 6 is part of the main MCU timeline so that makes season 7 ALSO part of it. We don't see sad and depressed people cause they moved on pretty fast or didn't lose anyone near and dear to them. About the time travel. The Time monolith was destroyed (Or converted to the energy thing) so they couldn't use that. Also almost a month after the snap the Avengers THEMSELVES tried to un-snap but couldn't. This would explain why S.H.I.E.L.D moved on in the first place and they were busy with other things. (Also Nick Fury snapping away did not help so they REALLY WERE busy). But the way Fitz and Simmons discovered time travel is kinda different. They had the Time-stream. So even though their main goal was to save the agents from the temple and stopping the Chronicoms through time, they probably did think about undoing the snap. But maybe they found out that the probability of them undoing the snap would be close to zero and the Avengers undoing it was pretty high. So maybe they decided to ignore the snap. Also now there is an interview with Ian (Fitz) where he says that Fitzsimmoms spent 4 years together which makes it 2023. So maybe the Avengers ALREADY brought everyone back by the time they decided to jump back to 2019 so they didn't have deal with the snap which made them concentrate on THEIR goal which is to save the agents and stop the Chronicoms.
In Runaways, Chase is time jumping from an alternate timeline where the snap did or did not happen. Even if it did, he's jumping from 2028 which is five years AFTER Endgame so he doesn't have to undo the snap. He wants to save Gert and he did save her but in doing so he erased the timeline from which he's from and this doesn't agree with the Endgame timetravel rules but Karl Mordo's comment about temporal manipulations in Doctor Strange agrees with this. So I guess it depends on the time machine created?
Wow that is a lot of text.