Actually, they do. My 4 year old female cat has peed on my lap once and on my bed once right in front of me. Both times was because of the new female cat in the house. She was pissed off. When we separated them, she stopped doing it. The day after we put them in the same room together, she constantly growled at the other cat, jumped onto my seat and peed on my leg. I'm at a loss as to what to do.Yea... cats don't have the same kind of emotions as we do. They don't retroactively think 'You did this, so I'm going to do that to you.' And even if they did, the cat would have no idea that peeing on things is a good way to get back at you. Cats don't pee on each others things to get back at one another. xD
Something is wrong, whether it be UTI, cancer, pain, who knows. Find a new vet/a pet behaviorist and keep trying.