Sure, but there's no reason that _has_ to be the case. Like, there've been attempts at standards; the volume is just never really there to make any of them _the_ standard.
Underground metro systems mostly started off as deeply weird and proprietary, as well (look at this one, weird gauge, tiny little trains, originally _cable_ operated: https://en.wikipedia.org/wiki/Glasgow_Subway), but nowadays they're _fairly_ standard, say.
Sure but they aren't and even if there was, for anything but single lines they are worse. Most cities already have a rail system and extending that is better.
Monorail operations aren't cheaper, at best their construction is cheaper. But if you need interchanges it gets more expensive again.
Its simple really they over very few actual advantages while having many disadvantages, so when they are built its usually because some monorail builder managed to convince some local politicians despite most experts saying they should just build normal rail.
Underground metro systems mostly started off as deeply weird and proprietary, as well (look at this one, weird gauge, tiny little trains, originally _cable_ operated: https://en.wikipedia.org/wiki/Glasgow_Subway), but nowadays they're _fairly_ standard, say.