One problem is that Excel uses locale settings for parsing CSV files (and, to be fair, other text files). So if you're in e.g. Europe and you've configured Excel to use commas as decimal separators, Excel imports numbers with decimals (with points as decimal separator) as text. Or it thinks the point is a thousands separator. I forgot exactly which one of those incorrect options it chooses.

I don't know what they were thinking, using a UI setting for parsing an interchange format.

There's a way around, IIRC, with the "From text / csv" command, but that looses a lot of the convenience of double-clicking a CSV file in Explorer or whatever to open it in Excel.