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.
As Wiktionary puts it:
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.