As for why Thor: Ragnarok might do it, we know for a fact that during the movie, Thor be separated from Mjolnir.
That said, let’s not forget that Avengers: Infinity War concept art showed Thor wielding an axe alongside Rocket Raccoon, not Mjolnir.
He’ll still have his amazing, “god-like” powers, but for a while he won’t be swinging around his trusty hammer.
The initial reason for Thor losing Mjolnir won’t be due to any severing, as some Ragnarok photos that have been released showing Chris Hemsworth in the Sakaar setting of the movie, and both his hands are in tip-top shape.