why would there be a magic character who relies on a real-world wheelchair when they presumably have magical abilities that would eliminate their disability,

Even moreso in something like a typical medium-high magic D&D setting, where most medium size towns have at least one person around that can fix those sorts of problems for people several times a day.

and why would that be someone’s fantasy?

Because to some people the most important thing of all is representation of specific groups and everything else is secondary.