There's a hint in the OP why Frieren doesn't live with the other elves, but they'll probably explain it in this run so I'll leave it off there. They've mentioned a few pieces of it already. (edit: there's actually two hints)
Dwarves don't live that much longer than humans, relative to elves.
You should be asking why Frieren was apprenticed to the greatest human mage who has ever lived.
Healing Frieren's detachment from others is blatantly her main character development. They say it right to the audience's face, over and over.