As Wiktionary puts it:

The word data is more often used as an uncountable noun with a singular verb than as a plural noun with singular datum.

It's like "hair". You can hold a single 'hair', you can also hold three hairs. But if you're looking at an entire mane, you ain't counting, so it's referred to as "hair" again.

I guess I tend to use data as a mass noun when referring to computer data ("there's a lot of data on that drive") and as a regular noun when referring to data in the scientific sense ("these data show xyz")

I have switched to just saying dataset