Because many who travel with pets prefer to hold their small well behaved dog on their lap, rather than stuffing them under the seat in a bag.
This is not a rant.
This is Flyertalk where we often discuss getting the most from our travel experience, and taking every advantage available.
I think most people who travel with ESA are more interested keeping the dog on their lap, not in saving $100.
This forum is about traveling with pets. Some Flyertalkers , who travel with pets, are interested in this subject. They may or may not want to draw attention to themselves here.