The ONT should be considered part of the ISPs network and not tampered with or replaced. Fibre with an ONT (PON) is a shared medium with other customers, and trying to use your own equipment has the possibility of degrading service for other customers.
Though there were throwbacks, like early iPhone, with special tariffs, but those were 95% for business reasons not explained to end buyer and 5% for marking the terminal in network as "the one that is slightly out of spec and remember to special case call redirection"
Why would a large ISP allow customers to bring their own ONT if it means higher chance of problems on the PON? It doesn't seem worth the reputational risk. Allowing customers to bring their own router doesn't affect other customers.