US universities have a lot of moving parts in their applications, so rejection often can't really be pinned down on one singular factor. Assuming you have written all 10/10 essays for every university, I would still not apply to most of these. I could talk way more about this, but your record is pretty average/slightly above average for international applicants (the UK is also not great for you in terms of contextual holistic evaluation since there's plenty of competitive applicants), and the fact that universities like Stanford, Penn and Cornell have need-aware aid packages basically bring your chances down to an extremely slim figure. You also underestimate just how competitive schools like MIT and Caltech are - tons of students with 1600s, 5A*s (or whatever perfect stat you can come up with), and 10 amazing ECs get rejected every year from these schools.
If you really want to go to the States, please pick your choices responsibly, and always have backups that you can default to. Good luck with admissions!