No I don't pay extra, maybe it's not a static ip address, but I have had it a long time, i would think they would let you know since I have an app to monitor some security cameras and when I tried to use it, it wouldn't connect. that's actually how i discovered it. i changed the ip address in the app to the new one and now it works. I was just wondering if it could be some sort of hack, thanks for your response!
As far as the "why" part, there are a couple of answers. Historically, before always on connections were common, it was just cheaper to maintain enough IP addresses for peak demand and assign them dynamically.
There's now the issue of running out of IPv4 address space as well.
Finally, and more cynically, it's another service they can charge for.