Those of us with DSL connections must suffer either an extremely limited selection of DSL modem/routers that can run Linux/OpenWRT, or have to suffer running a Linux/OpenWRT router behind a DSL modem (that often has proprietary and out of date firmware).
I put together some stuff so I could transfer PPPoE sessions to a backup system and then I could reboot the NAT boxes for upgrates with minimal downtime. Sometimes, it even worked ;)
I configured the provided gateway/router-widget to provide a "DMZ Plus" mode for my router (a custom box running Tomato or OpenWRT or something), and I called to get ports 25 and 80 unblocked. And then, plus-or-minus some completely-surmountable difficulty with making dynamic DNS behave properly it all worked fine.
For years.
I never connected anything other than my router to the ISP-provided device.
There's probably some corner cases where this configuration falls flat, but I never ran into them.
What might be some practical advantages of what you suggest?
Using half the power
Well okay, you shelled out $29.99 for a new shiny USB DSL modem. How many years should pass to at least have a ROI?
A device runs on electricity and performs a function. It consumes power at a rate of x.
You're telling me that a device that performs the same function will consume power at a rate of precisely x/2 simply by virtue of being plugged into USB?
As for why not USB specifically, probably because such a device is inherently much faster and responsive in upload to the Internet than downloads, and therefore it makes less sense.
They are pricey and difficult to come by, but with something like a Turris Omnia as a host, you _can_ have a single-device modem/router/WiFi AP
They definitely existed in the UK for a time and were often supplied by ISPs. IIRC they were only supported in Windows XP, and drivers were never provided for Vista.
ZyXEL if I remember correctly did make an ADSL2+ at the time PCI-E card. Literally just a DSL modem wired to a Realtek 8139 NIC. You could slap it in a Linux (or BSD, or Windows) PC and just use PPPoE to connect to the internet
Naturally it was impossible to order the damn thing and I never got to realize my dream of an "all-in-one" DSL Linux router.
After moving to DOCSIS (Cable) internet I ran into the same confusing problem. "Thankfully" with Fiber everything is just ethernet (more or less) now. But it was an infuriating time in the 2010's