People like the idea of doing something (in the case, becoming fit). They are not fit. This causes brain distress. Brain wants to do something about this dissonance, so brain says, "oh, I can buy a very expensive item that is a tool for becoming fit. Ahhhh, there is some relief to my dissonance." And then you buy the mirror and continue to not train, but your brain feels some sense of completion.

This factor has been studied. https://interruptions.net/literature/Wicklund-BASP81.pdf

This factor I have observed in my own behaviors first-hand.