The main reason is that each time I take a moment to "think" about how I feel, my mood changes accordingly.
In a way it's like a self-inflicted Hawthorne Effect when I try to consciously track my mood.
[caveats to the Hawthorne Effect: https://www.nber.org/papers/w15016.pdf]
Another issue is that I'm not sure what to do with the information such an app feature would generate. For many, mood is the result of things that aren't within their control.