I dunno. I banged out a simple one in React during COVID, to kill time. I piggybacked on Google Tour Builder so that I wouldn't have to make my own, but Google deprecated it.
I'd been inspired by the decidedly weak mechanisms used by the Rick Steves guides (hey, they're free, no complaints). It's a podcast plus a PDF map, and surely I could do better.
It worked fine, but it was just proof-of-concept. Adapting it to indoors would be a bit of a pain, but maybe that needs a different solution. (In a museum, after all, you can just have a sign that says "play audio file #7", which I have seen in use in several museums.)
I don't know why somebody hasn't tried to commercialize it. I did stick in stubs for a tour-guide tour, where people could contribute and charge, but I didn't flesh it out. Seems like a fine market to me.
