I have noticed this and it never really bothered me. For myself, I just don't want distraction during the liturgy and minimizing changes helps so we try to keep the same pew or close to it if it's occupied when we arrive. Also, the other families do the same and we don't want them to have to look for another place.

My supposition is that most people stay to the rear because new or visiting members usually sit there and just stay. If they don't start there and they're young, they end up there when they have to worry about a newborn crying or a young child misbehaving. My wife and I have commented on how our parishioners seem to creep forward as they age-- both to see and hear better and also to make room in the back for families with younger children.

Much to our surprise, we found our children's behavior improved markedly when we moved forward and they were able to see over the adults. Their ability to sit still improved just as much when their feet were able to rest on the floor while sitting in the pews.

What any of that may mean to anyone, I don't know, but they are all either truthful facts or thoughtful speculation on those facts. Whatever the actual mixture causing this phenomenon, it does amuse us.