Because "data" is the plural of "datum".
Consider a list of peoples' ages:
17, 22, 25, 32, etc.: each number is a datum.
The full set of these values is "data".
The data are 17, 22, 25, 32, etc.
It's useful in careful writing to distinguish the singular and plural, just like the difference between "statistic" and "statistics", where Latin need not bother you.
In one English translation, Psalm 137 starts "By the waters of Babylon, there we sat down and wept, when we remembered Zion."
In modern English, the term "headwaters" is the source of a river, usually stated as plural. '