Is there any specific reason an ISP will need to change your IP address? What is the purpose of a dynamic IP versus a static IP? Every 6 months or so, its common that ISPs tend to change your IP.
When ISPs were first starting, everyone connected to the Internet over a modem. And most people used the Internet for a few minutes to a few hours per week. Assigning a static IP to every subscriber would have been very expensive, for something that most people used just a few minutes a week.
As broadband connections have become more common, the practical reasons for not assigning a static IP have become much less noticeable, as now the majority of connections are “always-on”–even when nobody is (actively) using the Internet.
So there’s a bit of a historical reason not to use static IPs–customers are already accustomed to using dynamic IPs.
When modern ISPs enforce dynamic IPs these days, it may be in part to distinguish between “consumer” and “professional” services–by reserving static IPs for customers who pay more, it gives customers who need that feature an incentive to upgrade their service level.
It can also serve as a deterrent for people abusing their consumer-grade service. Many ISPs, for instance, explicitly prohibit running “servers” on a home Internet connection. If every home user had a static IP, they’d be more inclined to abuse such terms of service.
It’s also less of a management problem to assign customers dynamic IPs. If you move across town (but within the same ISP’s service area), there’s no need to re-assign how your static IP is routed; you’ll just get a dynamic IP that exists in the new neighborhood.