There is no monopoly, see MegaBus, FlixBus, OurBus, many others, not to mention specific city to city routes. For example, I used to live in the Minneapolis/St. Paul, MN area and there are specific shuttle busses to go to Rochester, MN to go to the Mayo Clinic.

Also, remember the US is huge. The bus usage in a country like Costa Rica (the closest non-US country I have experience with) is very high, but you can go the length of the country in a day. That wouldn't even get you part way across the US, so depending on where you're going it's not even comparable.