We're less than eight months into the Biden Administration, and every single day has come with it some new fuck-up that causes it to drop trough on either the domestic or international stage. Can you say, if you were in her shoes, that you'd have a single fuck left to give?
I could be wrong, but given Psaki's career path, my guess is that she hit her personal ceiling. Connected enough for a post, but not enough to be part of the bigger slice of it all, she's probably face-to-face with the fact that this is as good as she's ever going to do. Under difference circumstances she might be placated, but knowing that "they" are holding her back can easily turn loyalty to bitterness, and give her every reason to take revenge on a system that let her down.
It is, of course, also possible she's fucking incompetent. You decide!