Why do we blunder?

Why do we blunder at chess? "Because we can" is one possible answer. "No idea" is probably just as good. But my favourite reason is the streakers defence. "It was a good idea at the time."
Here is a game from a Correspondence event I am currently directing. The reason I picked this game is that it is played between two reasonably strong players, and as a CC game, you would think the chances of an outright mistake are quite low. But on move 25 Black spots what looks like a good move, only to have it all go wrong two moves later. 27.Qd3 looked pretty obvious, so I'm not sure what Black was thinking. Maybe he saw a defence which had a flaw in it, or just decided that winning the exchange was good enough. It turns out it wasn't.

Jones,,Kenneth - Zhakarov,Oleg [C92]
Australian Candidate Masters, 2018