Sure, there are some edge cases where guessing the separator would be hard, but in 99% of cases you can correctly identify the separator pretty easily and in case you can't you just don't pre-fill that field in the import dialog.
Psst, the joke is that it's a "comma separated variable" file.
The V in CSV doesn't stand for "variable". And that wasn't the joke.
If the file ends in CSV then the comma is the separator (the C is CSV).
What is the separator in this .csv file ;)
1,2;3,4;5,6
But on a more serious note, I completely agree. There are so many small quality of life things that can and should be fixed in Excel, and it's baffling that they haven't.