The bus companies are not allowed to do random stops en route.
The stops have to be scheduled in advance, and they don't schedule many stops. They actually mostly follow these rules.
The result are fast point-to-point routes, without stops at every palm tree. For example, Sombat Tour to Surin runs from Vibhadhavee all the way to Surin province without stopping in any town.
Buses to and from Ubon usually do have scheduled stops, I guess otherwise it would be too far.
Buses to Khonkaen usually (eg Chantours) use the Korat bypass, they don't enter Korat. When buying a ticket one can ask whether this bus stops at Korat. 20 years ago, every bus to Khonkaen would stop at Korat - no more.