>Is that because wireshark can't do that just from packet captures?

Well, not quite. I think it's more that nobody has taken the time to implement it. That's not to say such an implementation would automatically decrypt the traffic from a capture with no extra leg work, of course. Wireshark dissectors have user configurable preferences, and presumably this would be where captured secrets could be set for use. This is how it handles TLS decryption [1], which works beautifully.

[1] https://wiki.wireshark.org/TLS#tls-decryption