Numbers in Excel are limited to 15 digits, no matter what notation. If you want a number-looking string yo be a string, you can quote it with a leading ' .
I would posit there's a near zero likelihood that someone copy/pasting a, say 21 digit number, wants it to automatically convert to Scientific Notation and lose data.
Excel should do the least astonishing thing when someone is working with data. If it cannot handle 15+ digits, it should assume a string, not a broken number.
Like the northern star, a refusal to waiver under any persuasion.