> Why put in all that effort when the map won't even show it though?

Just select a different layer, the "Transport Map" renders bus routes: https://www.openstreetmap.org/#layers=T

This is a problem with all maps. You cannot show everything, so you reduce it to something you want the map to show. While things like waste baskets, hydrants, dog excrement bag dispensers, etc. Can reasonably show up on deep zoom levels because they're just small icons, I think bus routes can clash with plenty of things that many people would also consider important.