(I had to look up the meaning of "ONT".)
Local ISPs (to me) all do use routers, like Inteno
https://www.intenogroup.com/products/gateways/eg300/ that have both RJ45 and SFP slot (WAN-side).
That is thus a "ONT-router combo". I.e. they are "in use", just like the other combos, in some parts of Earth.
We have both fiber and Ethernet copper to apartment. There was an option to connect via fiber; the ISP was happy/obliged to offer "fiber-Ethernet adapter" (an ONT?) for $100, but only the copper is connected (for now), so we have effectively FTTB. Majority of local ISPs' customers are probably FTTB, with TV-cabling (prominent), DSL, or Ethernet for the final yards. Therefore, while the router models could do ONT and buildings had fiber to apartment, hardly anyone actually uses that feature. (Not sure if the ISP's ONT on the building works for fiber to fiber too, or would they have to switch their device should the customers demand fiber.)
I would be happy to plug a fiber into my tranceiver in SFP-slot of my router, if I had one and ISP were okay with that (big IFs). Separate ONT is almost as good. Games have "combos". Networking is Serious Business and no game.