Behavior of some people on public buses( For school )

Noticed how on public buses, there is usually at least one person who takes up two seats? I am not implying that they are fat, what I am actually saying is the selfish way they behave. They purposely sit on the outside, hence blocking the way to the seat inside. Most people would not go forward and ask for that seat but would instead leave things as it is unless they really need that seat. Furthermore, even if you ask for that seat, these people would not move in but would instead shift their legs and leave you to squeeze through the narrow passage to the inside seat. This is especially difficult on a crowded bus. I remember once, when I was coming back from school and it was the peak hour, I boarded a bus(852) and it was very crowded. I noticed that at the back part of the bus. There were one or two empty seats that were blocked by the legs of these selfish people. One of these people moved in to let another person seat but the other, a woman, seeing the crowd, refused to move in and just sat there as though she was the only one on the bus. I am sure that you would, seeing the crowd, move in and let other people sit. This is not the only case. I am sure that there are countless of similar people who do this too.
What I do not understand is why they refuse to move in and let other people sit. Is it because they have a sickness and does not want anyone to be infected too? Or is it because they belong to some kind of rich family and feels superior to others? Or is it because they are plain lazy. No person is superior to another. All are equal. Being rich, famous or important is just a privilege and does not warrant you to look down on other, less-privileged people. We should maintain a good habit of moving in and letting others sit, and not be some kind of lazy, snobbish person.