As others have pointed out, in other countries the wealth gaps aren't as large, and for example in Chile, the bus system is relatively luxurious (and a necessity) for even average passengers. Probably due to competition as well.
In the US it seems, the cost of vehicles, drivers, overhead staffing, etc. is large enough that they cannot but make the experience horrible -- in terms of routes, seating configurations, schedules.
And as a result there are 2 issues with buses that just make me swear off them, not because of the other passengers or anything:
1) The stop and go nature of navigating in cities just to get to the open road, or to make any kind of stop, and the jostling of the highway -- you just cannot feel at ease or relaxed that the journey is underway and calm.
2) The back-breaking discomfort of the seats that Greyhound installs to reach the passenger density needed to break even. Knees crammed against the hard plastic back of the seat in front of you, and no recline for 6 hours. In other countries, they have buses with sleeper seats and cabins for godssake.
I will go out of my way to take a train even if it's slightly longer, to avoid buses.