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FWIW PKI tends to mean a central point of failure. Some Russian organizations can't get TLS certificates because of sanctions.

Pki here does not mean a global CA. You can run your own CAs (and should).

Since only two parties are involved, why not use the easier pre-shared key system in that case?

For the many reasons I listed. Pre sharesd keys are almost always global and you can’t do forensics to find the leak.

You can cross trust and establish alternative trust paths in PKIs