Because that is the terms Verizon decided and you agreed. Is it designed to increase profit? Of course. Can they do that? Of course. You are given the use up to that amount data for that month.
bobbyg51, apparently Verizon doesn't use roll over plans. Some years ago when I had AT&T they used roll over which basically sounds good, but just a marketing approach. I never ended up using the roll over that accumulated or any of my friends that had it. Because of already choosing a plan that fits your needs. It's nice to say oh I got 5000 minutes or 800 GB of data accumulated, but had no use for it.