Phone numbers aren’t exactly unique. It’s really not much different than being assigned a static IP address from your ISP. They’re assigned and if a line is cancelled or you change your number, it goes to a dormant state for a while then is reassigned to someone else.

Your phone’s IMEI on the other hand is a unique number, similar to a MAC address for network devices. Unlike a MAC though, it is illegal to spoof or clone an IMEI. Infrastructure however wasn’t designed to use the IMEI or MAC as the publicly accessible address, it was designed with a middle translation layer in mind.

Not 100% sure, my early history is lacking a bit, but I think that was simply because the fundamental network design underlying everything we use predates unique identifiers like MAC addresses existing.