Lots of people have them. You don't need to be in a wheelchair to need a handicapped parking placard, for example. An extra hundred feet of walking can be a major hurdle for some even if that isn't obviously apparent. I don't know if you really need that "emotional support" animal on your plane flight or not, and I'm not going to ever assume I do.
But I do know that some people abuse these things (because I know people who have), and the price of that is it makes it that much harder for people who actually need them to make use of them. Don't claim a medical condition because you just want to bring your damn pets with you.