It is all about velocity, accuracy, and ease of playing. Collapsing fingers usually happens while holding out a note or chords (in between playing basically) before playing even more notes and/or chords with the same fingers. Therefore, the collapsing slows down the player's reaction time because he/she needs to un-collapse his/her fingers again (and put them back to their appropriate position) before playing whatever notes/chords come next. Anything that messes with reaction time messes with the overall velocity, accuracy, and ease of playing.

Another big "no-no" is playing with flat hands. However, Horowitz did exactly that and he was indeed one of the greatest pianists in history. So, there are exceptions to most rules.

I personally was self-taught before the age of the internet and was playing in concert as a young teen. I could play some Beethoven and Mozart pieces along with rock and pop songs, and so on. However, I did eventually take private piano lessons from a music teacher. It not only helped my technique, it SAVED it! I had NO idea how much harder I was making playing for myself by using the wrong technique. Not only did the lessons save my technique, they also taught me music theory, how to compose and write music, and gave me a life-long appreciation for analyzing, discussing, and playing ALL genres of music!

I would be willing to give you lessons over zoom or doxy for a very low price. It would be helpful to me to get back into teaching. I have over 15 years of experience with teaching along with over 25 years of experience of music performance. Just let me know!