Because it is undesirable. It is very easy to open a port and relatively easy to get dynamic dns account.

So everything that p2p offers is easy to replace by normal Internet Protocol.

But it is impossible to make a scalable network with good UX when every user has to be a devops engineer.

Discovery and search are also physically impossible to do in p2p, beyond simple distributed hash maps. You can't do distributed Google, or distributed notifications or distributed counting of likes or distributed threads. All of these need centralization.

For 1-on-1 stuff like private chat rooms, things are much simpler, but again, you need every user to have their own always online server. That simply won't happen. Civilization is entirely about division of labor. People can't do everything on their own.