It also doesn't prevent a human or other system doing:

cat input1.csv input2.csv > output.csv

resulting in a single file containing multiple formats.

Also, what variant is this:

    1,5,Here is a string "" that does stuff,2021-1-1

What is the value of the third column?

Is this a CSV file without quoting? Then it's

    Here is a string "" that does stuff

Or is it a CSV file with double quote escaping? Then it's

    Here is a string " that does stuff

This is fundamentally undecidable without knowledge of what the format it is.

You can decide to just assume RFC compliant CSVs in the event of ambiguity, but then you absolutely will get bugs from users with non-RFC compliant CSV files.