Not every single carrier supports eSIMs, no, however in the US the current providers that support eSIMs are:

AT&T

Boost Mobile

Caroline West Wireless

Cellcom

Credo Mobile

C Spire

FirstNet

H2O Wireless

Nex-Tech Wireless

PureTalk

Red Pocket

Spectrum Mobile

Straight Talk

Strata Networks

T-Mobile USA

Tracfone

UScellular

Verizon Wireless

Xfinity Mobile

So most of them. Plus more will provide them if Apple continues to roll them out.

And for the ones that don’t, you can use an app that acts as a SIM for that network, allowing you to use an eSIM.

The only real technical reason for carriers to keep physical SIM cards is to dissuade people from switching networks as it’s more of a hassle.