mains

Why is it that some people are "Rein" or "Ana" mains, but for the most part, there's non-specific mains like DPS? I get that Rein and Ana are a common staple of the meta, but what about off-tank heroes like Roadhog and Zarya? People are still considered mains for that, yet we still refer to them as tanks.

There are 5 roles teams usually fill:
Hitscan DPS - mccree, soldier 76 etc
Projectile DPS - phara, genji etc
Main tank - Reinhart, Winston
Flex - basically anything but usually zarya, roadhog, d.va
Support - ana, Lucio etc.

I'm kind of confused by your question but I hope this helps

Don't use the roles prescriptively, they just happen to describe most setups. If your best three heroes are in three different roles, and so are your teammates', then you'll be switching roles a lot if you hope to win.