No, it's Excel trying to be too clever. It does the same thing with manual imput if you don't proactively change the field type.

You can import a DSV into Excel without mangling datatypes in a few different ways. Probably the best way is using Power Query.

A DSV generally does have a schema. It's just not in the file format itself. Just because it isn't self-describing doesn't mean it isn't described. It just means the schema is communicated outside of the data interchange.