I don’t think most people are late on purpose. Like I said I’m not a person who runs late so I can only guess. But I would say that when people are late it is taking them too long to get ready. Or perhaps they forgot to buy a gift for the hostess so they have to stop off at the store.
Another reason would be that you happen to know this host/ess and that (s)he will not be ready on time and you want to allow them time to get their crud together.
And when it comes to a general party/bash there is no reason to arrive ‘on time’ as the nature is to have people coming and going at will. I would guess many people don’t even know what time the party “starts”. But I quite going to these type of parties a few years back.