Why Isn't Post-Quantum Encryption More Widely Adopted Yet?

A couple of weeks ago, I saw an article on "Harvest now, decrypt later" and started to do some research on post-quantum encryption. To my surprise, I found that there are several post-quantum encryption algorithms that are proven to work!
As I understand it, the main reason that widespread adoption has not happened yet is the inefficiency of those new algorithms. However, somehow Signal and Apple are using post-quantum encryption and have managed to scale it.

This leads me to …

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