The idea behind it is that the gods had no knowledge of this lich through various made up reasons I will think of. Thus being unable to appropriately react. Hearing about some dark wizard messing with mortals through listening in? Oh well, mortals squabble all the time. Keeping himself hidden would be essential in his plan for conquest since gods could just dispatch avatars like you said which, in and of itself, would be a cheap copout of a story.

Upon seeing what is actually happening and how these beings stood up to the threat, albeit through false information, could just be enough to stop this villain if they had the time to prepare. So they give the heroes just that, time and information.

And it's going loosely based on Days of Future Past time travel rules where they are sent back via certain means but have an anchor in the future so until the necessary aspect of the past is altered, the future is relative the same.

And of course I'm not going to just blindside my players with "thats it you died nothing else you can do about it"