system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU-Loki] How and why do the ________ allow the TVA and other time travelers to (spoilers)?

This question also applies to the 616 MU as well:

How and Why do Cosmic Entities like the Celestials and Galactus and others allow the TVA and other mere mortal time travelers to 'reset' the timelines/realities they are in?

There is no way cosmically aware entities like Galactus, the Celestials, Eon, and others would not be aware of events in their own past being changed, or that the timeline they are part of is being pruned or erased or whatever.  And with their power, they could easily intervene or prevent it.  So why do they allow it?
gpt
Aside from Ego, we haven't really met any entities of such scale in the MCU.  Ego, for all his power, didn't seem particularly aware or interested in the multiverse, and the same may be true for many of his stature.  

The only group we see take an active interest in the multiverse is the Masters of the Mystic Arts, and they are primarily concerned with incursions from other universes, not necessarily other timelines.  While they almost certainly would take arms against Kang, Kang comes from a thousand years in the future. His technology is likely more than a match for the sorcerers of the 21st century.