"Modem" stands for "modulator-demodulator" and it is there to convert the TCP/IP traffic in your house to some sort of non-TCP/IP connection that goes to your ISP. In case of cable provider, I believe the outgoing protocol is called DOCSIS, and in case of a telephone provider it could be ADSL/VDSL etc. (Historically some also used ATM above the physical layer.) The modem is there to do that translation.
With fiber, there is TCP/IP traffic within your house and TCP/IP traffic to your ISP. There is no translation to do, so no translator device (modem) is needed. The only thing needed is a physical layer conversion, from 0s and 1s as voltage over copper, to (exactly same) 0s and 1s as light over fiber. This conversion (not "translation") could be done by a stand-alone ONT (a rather uncomplicated, or could I say "trivial" device), but there are several router boxes that have fiber connections — either built-in (on board) or as plug-in modules.
Different ISPs have different offerings, but as you might imagine not many people want to manage their own router. My ISP has provided me with a free ONT which goes into my router, to which I connect my WiFi access points, all of which I manage myself.
With fiber, there is TCP/IP traffic within your house and TCP/IP traffic to your ISP. There is no translation to do, so no translator device (modem) is needed. The only thing needed is a physical layer conversion, from 0s and 1s as voltage over copper, to (exactly same) 0s and 1s as light over fiber. This conversion (not "translation") could be done by a stand-alone ONT (a rather uncomplicated, or could I say "trivial" device), but there are several router boxes that have fiber connections — either built-in (on board) or as plug-in modules.
Different ISPs have different offerings, but as you might imagine not many people want to manage their own router. My ISP has provided me with a free ONT which goes into my router, to which I connect my WiFi access points, all of which I manage myself.