[icedchai]

Telecom legacy, probably. The ONT is seen more like the old "demarc" / "network interface device" from the copper days. You didn't install your own demarc: that was done by the telephone company. Everything after the "demarc" is customer operated, everything before the "demarc" is telco operated.

Operationally, it also simplifies things having a relatively uniform set of ONTs. With cable modems, there are 100's (1000's?) of models that have to be evaluated, tested, certified. Cable companies often do their own firmware updates.