I’m going to innocently guess it’s because the tips are small parts put under duress and with more use, the looser they become and just… Fall off? And then they aren’t seen on the ground of the station because people drive over them and they get scattered or smashed on the street. Of course I’m still learning to drive (in my mid-twenties) and my family members who are licensed are more often the ones who get work done on our cars. I also live in a state were self-pumping isn’t that common.