How does it give customers a better choice?
With a dedicated ONT you can use any ethernet router, if it breaks its the ISPs problem not mine, compatibility is guaranteed.
If the ONT is built-in then you're talking a much more specialist item. Even if they become more common as FTTP becomes standard, there's another problem.
So you bought an ONT gateway, it has GPON, you want to upgrade your package and oh dear, your network wants to move you onto XGSPON. If you spent a fortune then maybe you can swap your GPON SFP ONT for a XGSPON SFP+ ONT, if you didn't then oh dear, new router time.
In my configuration with the Openreach ONT (max 2.5W), pfSense box (~11w) and dedicated Access Point (not sure the power consumption), I have a better configuration than an all-in-one. If using your own ONT becomes a thing, odds are standalone ONTs will die off so I'd have to use a gateway/router all-in-one in bridge mode, which would use a LOT more electricity than a standalone ONT as you can't generally turn off the power to the components you don't need like WiFi.