Easy!
Try and put yourself in the situation...
You're standing in line holding your baby, or nursing your sore knees or bum hip. The bus pulls up. The line moves forward. You haven't visited before, so you're not able to eyeball the exact number of bodies to seats. And anyway maybe someone was holding a place in the line for the rest of their party, because grandma has to sit. Or maybe a wheelchair bound person and their entourage had to be loaded first. Who knows? In any case, you follow the crowd onto the bus. When you GET on, there are empty seats - but people are grabbing them as fast as they can.
You get to the back of the bus and realize it's full. You turn to try to catch the empty seat beside you, but you're not moving quickly and a teenager has already parked his butt in it. Suddenly there's no seats left at all. You'd like to get off the bus at this point, but all this time more people have been packing in, and there's nowhere for you to go.
So you and your hip surgery have to suck it up and stand and try not to be annoyed at the teenager who grabbed the seat next to you. After all, he's surely got a hidden disability that entitles him to that seat.
And on the plus side, you can always use him to cushion your fall, in the event the the bus driver has to suddenly stomp on the brakes. Or you can drop your baby on him. It's all good.