That occurs because too many pax are incapable of logical thought. How many times have you tried to dispose of a paper towel in the lav bin only to see it full because others have barely used a towel and just dropped it in bin so that it takes up more space. Or, a barely used towel half sticking out from the bin. Too many people are slobs. The simplest solution is to crumple the towel into a small ball and drop it in the bin. You’d be amazed at how much more space that would provide. I’ve been on TATL flights where people seemed to grasp that concept and the bin never filled up. (Of course, I was the only passenger on that flight. JK! 😂 )