There are several reasons. Some are good reasons, but I think most are not.
Some good reasons is that people have gotten sick during mass and had to leave early. Other times it might be work or medical personnel on call all weekend. I know that there are some people who are very judgemental on medical personnel about this and think that they should not answer an emergency call/beep until after mass. Although… that is until their own loved one has the emergency. They are also sometimes the ones who are the first to sue the medical professional - especially when they discover that if he/she answered the call a half hour, 15 min, 10 min earlier, their loved ones would have been ok. But I digress.
When I’ve seen people leave early, it’s usually after communion. I know a few who do this and it’s their belief that they stayed for the most important part and the rest of it is just announcements that they don’t care about. Then I think the major one is what someone else mentioned. Many don’t want to wait in line to get out of the church parking lot. They have this mindset to push and get out before everyone else. It’s not a good reason in my mind, but I know that it is the reason for many of them. My family usually just waited in church for about 10 to 15 minutes longer after mass. Then we went out to the car and the parking lot was fairly clear. My Dad’s mind was, “Why sit in a car when we can sit in here and pray?”
Another reason is that children’s sports teams schedule games every single weekend and usually during the most inconvenient times. So, families will leave mass early to get to the game on time. I’m not saying I agree with this, but I know that happens.
At my parish, we’re in the city, so I don’t see the mad rush to get out as much as I have in the suburbs. There are a few that leave, but many stay to pray or to walk around the cathedral or take the tour of the cathedral afterwards. Others want to stay and hear the organ postlude. But I think because many of the people in the congregation are visitors, they tend to make a point to come to our parish for mass, whereas at home parishes, that would be a different story.