In Arena, you start humbly, but rise high enough that you destroy Jagar Tharn, one of the most powerful mages in the whole of the history of the Elder Scrolls games.
In Daggerfall, you're the right-hand man of the Emperor, and seek out a world-ending superweapon, and may just take it for yourself.
In Morrowind, you are a god, or very close to. The reincarnation of the Nevarine. That you can become overpowered makes sense. Bandits should fear you. Ordinary people should.
In Oblivion, you're the right-hand man of someone who ascends. You walk into literal Hell and fight demons. The bare mysteries of the world are laid clear by you. You steal the book of spells of immortal characters.
In Skyrim, you're descendant from a man who became a god. You have a gift that can shatter the heavens, and break open portals between worlds. You kill the son of Akatosh.
You're right, it is design, because it makes sense.
What does this mean?
Bare as in raw, foundational, etc.
In Morrowind I used this to create an extremely long-lasting fire spell dealing 0 damage to self. The result was a spell which cost barely any mana and resulted in you walking around engulfed in harmless flames. A really cool visual touch.
I do distinctly remember being unable to recreate that one fire spell, though. Perhaps it was not possible to set damage to 0 in Oblivion? That was part of why the spell became so cheap and allowed for extreme duration.