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.
And the word “data” is only one word, so “data is plural”