Because carriers and hardware vendors don't want to play ball. Some carriers consider the user's modem as part of their network and want to control it, hardware vendors want to keep their precious firmware blobs and private APIs to themselves.
Linux has all the drivers and infrastructure in place to use cell networks efficiently, carriers and vendors just don't want to be part of that.
Source: my only Internet access at home comes from a LTE router running Debian I built myself.
Unfortunately I never got around to writing it down and as it has been running pretty much unattended for the past three years it is not fresh in my mind.
The gist is that since it is a router only (switch and access point are external appliances) it is not that difficult, there are plenty of resources on the web explaining how to turn a linux box into a router. I had to configure a bit of nftables, dhcpd, dnsmask and stubby and that's pretty much it.
The server is a standard PC Engines APU4 and the modem in the mPCIe slot is an Huawei ME906s.
As I alluded earlier, the hardest part is finding a good modem and getting it to work. Once the modem shows up as a network interface the hard part is done.
Linux has all the drivers and infrastructure in place to use cell networks efficiently, carriers and vendors just don't want to be part of that.
Source: my only Internet access at home comes from a LTE router running Debian I built myself.