People make a lot of assumptions about disabilities and what qualifies someone to use a handicapped parking spot. There are a lot of physical disabilities and sources of chronic pain and difficulty that are invisible to the naked eye, so unless someone is overt about stealing space from a disabled person, it's best to not assume they don't need it themselves.
Sadly, because of misinformation and a culture that still systemically erases the diverse realities of disability (both physical and mental), the general public has a very limited image of who is disabled.
If you appear "young and healthy" it's not uncommon for people to accuse you of faking it, or not having a valid ailment, which opens up a very complex can of worms.
AITA for /how/ I told someone I would not give up my handicap spot?