Bus Muppets

I was on the bus the other day and it passed a number of people waiting at a bus stop because the driver believed the bus was full. The bus wasn't full. In fact, there were even seats but this was a reasonable conclusion for him to come to because there was a big crowd of people bunched up the front. I guess some people prefer to stand. But why don't they stand then at the back so that more people can get on the bus.