Why buses doesn’t have restroom/s but airplane does?
Many buses do have restrooms - most charter buses and buses that provide long distance service do. But you can always stop a bus for a bathroom break - you can't do that with a plane.
You can stop a bus. You can't stop a plane.
Many buses do if they're meant for long distance trips. The reason is maintenance. Obviously when you use the bathroom, they need to put the waste somewhere and refill the water and things every so often. They can't just dump it on the streets. Airplane flights are generally a few hours, but the buses you describe seem like they're meant for shorter distances. Charter buses and greyhound have bathrooms usually.
Long-distance buses generally DO have a toilet on board. (I promise you, nobody 'rests' in them.)
Schools use school buses for field trips. School buses do not need bathrooms as they are usually used for short time periods, Even a field trip ride is usually no more than an hour. Charter buses that go a long distance do have a bathroom.
Some busses actually do have bathrooms.