I suppose for online games, it's been sort of standardized that a "classical" time control for a long game is 90+30. I'm not saying that's an official online standard, but it's probably the best you can get with a single time control that mimics a traditional OTB match between 2 grandmasters. Of course, players can stipulate any time control they want, if they want a longer time control, like 120+30.
I don't know if it's practical to ditch multi time controls for OTB matches. The reason to have multiple time controls is to have a built in sudden death, so that the game won't go on forever. It's less of an issue for online games, since people are playing, usually, at their house, and can schedule games at whatever leisurely pace they want.
Amateur boxing rule: 3 rounds.
You don't see pro-boxers going around fighting 3 rounds.
My bad. You do see pro-boxers fighting 3 rounds, against 5 opponents (like a simul in chess).
This is why they don't do it. It looks less professional.