Kashering

Question

Dear Rabbi, With regard to kashering: Why doesn't a person need to kasher counter tops because of the concern that a hot food might fall directly onto its surface, especially on pesah where ashkenazim don't rely on the idea of notein taam lifgam for stam keilim? Shouldn't ashkenazic Jews follow their prevalent minhag, based on the Rama, and do hagalah on all utensils that have been used as a kli rishon, even if it was only one time? If you can kasher a pot by leaving the lid on top and letting it steam up, how come the Shulchan Aruch and most people today advise to either place the pot in a bigger pot or put a rock into it so it will boil over? Your idea seems a lot simpler so I wonder why I haven't come across it in Shulchan Aruch?