From the introduction:
“S.H.I.E.L.D. director Nick Fury sends a dire message from the future: multiple dimensions have collided and our reality is shredding apart!
“You must gather the Marvel universe’s mightiest superheroes and villains, and prepare for the final battle to save our timeline!”
...This may be the 90,000th time you’ve fought this “final battle”, given that you’re stuck in a time loop. You must fight continuously against thousands of enemies, including some with the power of Thanos himself, and you’re coming in as the equivalent of a level 1 scrub.