Because

1. When the internet was rolling out, a decentralized, open, best-effort solution of TCP/IP thankfully won over telephone companies’ centralized system
2. IPv6 is still not universal for some damn reason
3. Onion addresses solve these problems but good luck getting everyone aboard with Tor
4. You always trade anonymity for reachability, and with the amount of threats, NAT and firewalls have been put up to make it harder for unsolicited requests to reach you by default