Why are IP addresses for hosts dynamic?

Answer 1

Most people set up dynamic addresses because they don't want to get spam mail. With a Static address, evertime you go somewhere a "cookie" is saved in your computer. People track these cookies and send you spam mail. With a dynamic address everytime you go on-line you are using a different path to get connected, making it very difficult for people to track you.

Answer 2

This is usually set by your ISP. Some ISPs allow you to have a static IP address but it costs you more. The dynamic nature of your IP address makes it so that the ISP doesn't have to own a large number of IP addresses to provide you with internet service.