Just because there's magic doesn't mean it's evenly distributed. And finding someone capable of casting higher level magics isn't an easy feat.

I play in a slightly modified DnD 5e Forgotten Realms (some years in the future with a new Mystra)

Basically Regenerate would bring you back to your normal state or the state you perceive as your normal state.

Some examples:

You're born without an arm, if someone cast Regenerate on you you wouldn't grow a new arm. That arm was never there. To get that arm would take a True Polymorph. Which is not only a very high level spell, it's really not easy to find someone who could cast it.

In the case of the blind man the party met, they were blind for decades after losing his eyesight saving his family. He had fully accepted it about himself. He had no use for sight in his mind after living so long without it, it was a part of him. He perceived his normal self as blind. So if someone cast Regeneration to save his life he’d still be blind. (And someone had years before the party met him, but that's a story for another time)

Basically: learning magic is hard, the components are typically expensive, and finding someone who is already skilled in magic enough to cast it is hard. Not to mention the costs associated with it.

"Go to the nearest big city, there's bound to be someone who can." Yes but would they be willing to? How much would they charge? How long would you have to travel to get there? Is that feasible?

"What about Lesser Restoration, a second level spell? That one's easy to get to." Maybe in a big city, but out in a rural area that would likely still be tough as someone has to have the necessary prerequisites to get to that. You have to hone that craft and learn it from somewhere.