You already know why.
A laboratory was performing high energy studies which accidentally ripped a dimensional hole, tossing a whole bunch of random humans from earth into the Aether.
Once you were tossed into the Aether (think astral plane) you discovered that you are an extreme powerful Magus who was formerly starved for mana on earth due to it being a non-magical dimension. When you fell into the void of nothingness of the Aethereal plane you instinctively created a demiplane to house yourself.
Due to doing it instinctively without training you wound it up far too tightly, accidentally made it into an anchor, and failed to allow a method of disconnecting. So now you and anyone you bring in are trapped inside (barring very short dimensional hops that end with you being snapped back in by the Anchor effect).
You already know the answer.
"roughly several decades of magical education". Which is a guesstimate of how long it will take you to train to archamge status and learn the skills necessary to safely unravel your demiplane.
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Personally I am very satisfied with this game's plot (or lack thereof).
Not every game has to be "Journey to Mount Doom to
slay the Demon Queen"