one of these days, I will build a time machine, the first thing i'll do is track down the 14th century serf that discovered onions weren't poisonous and inject his ass with black mamba venom
I couldn't agree more. Also, as mentioned above, there are many people nowadays with a peanut allergy. It'd be extremely difficult to make sure that there are not only peanuty burgers but also ones without the slightest trace of peanuts in and on them. You'd need two kitchens, two counters, different plates and cutlery, different ovens, frying pans, dishwashers, and 2 sets of staff. It'd be too expensive, compared to the tiny profit they could make.
I love fried eggs on burgers. The yolk has to be runny so it bursts on the first bite. Don't think I could say the same thing about peanut butter, though. It just doesn't sound right, although I do think I did see PB on a burger on an episode of Diners, Drive-Ins, and Dives on the Food network.