That would fly against the assumption of most people that the router also is the 'modem' for one, and that the necessary 'modem' for whichever access-technology is often mandated by the ISP, for another.

Which you rarely can escape outside of business-contracts, regardless of concepts like 'router-freedom'.

In very rare circumstances you'll get to know the exact specifics of the low-level technicals before that shit is installed, and can plug in you own stuff without excess gear.