There are even FlixBusses in the US. I see them pretty often.

There are a number of nation-wide and regional inter-city bus systems. They're just not extremely popular since car ownership is typically pretty high, fuel costs are generally low-ish, and chances are when you get to your destination you'll need a car anyways.

See companies like:

https://vonlane.com/

https://www.vamoosebus.com/

https://www.luxbusamerica.com/