Esim activation in United States

Why don't you ask US Mobile, their choice to not support eSIM has nothing to do with Apple.

Doing some Googling would suggest that they do support eSIMs but you still need to contact them and ask what they support.

We don’t know why Apple is offering (only) specific carriers with iPhone purchases, though full-price purchases directly from Apple are carrier-unlocked, and US Mobile does offer eSIM.

There is (usually) a chat function available here (and more carrier info), and you can use that to ask Apple directly:

… iPhone - Buying iPhone - Apple

Let us know.