I’ve been to many parishes and it is true that some do and some don’t.
As for the reasons, I’m sure that varies. I think a lot of it is just going with the flow. If that’s what the parish always does, people will clap just as a matter of routine.
If people were applauding for Jesus and the Sacred Mysteries, I would have less of a problem with it. I’d venture to guess, though, that if you took a poll of people after Mass and asked why they clapped at the end, you would get a lot people shrugging their shoulders and admitting they really hadn’t thought about it before.
Then again, if you took a poll of people after Mass asking why they stood of knelt at certain parts of the liturgy, you would probably get the same blank stares. Lots of Catholics just don’t think about the reasons behind the actions of the liturgy. And if you don’t give it much thought, it’s not very likely you will know the difference between what liturgical actions are part of the Mass and what actions are just some tradition that crept into the parish experience over the years.