I haven’t read all of the replies to see if somebody else had said this, but it’s because the Internet was designed to be completely decentralized, whereas the phone system requires your line or device to be registered with the network operator(s). Any device that can get a valid Internet address for the local network can communicate with the whole Internet, but a phone will only work if it’s explicitly known by the phone service provider, and that information shared to all providers.

We could set up a system, layered on top of the Internet, by which each computer could register itself in a central directory each time it connects, and thus be reachable at the same address no matter where it connects, even on a NAT connection. In fact, it’s easy to do with a VPN and Dynamic DNS (both of which require the cooperation some centralized authority). It’s just not universal, because, well, what’s the utility of doing so?