Some school buses do have seat belts throughout the entire bus. For the most part, buses do not have seat belts. They tend to have seat belts in the rows closest to the front end. It depends on the...
Because vehicles over a certain size aren't required to have or use seat belts, so it would be a waste to put them on.
It is also a safety issue. The color stand out more.
It stops you flying out of your seat through the windscreen and into the other vehicle or object. A few bruises on your chest is nothing to a face of glass and possible death.