Why is maths plural in the UK, yet singular in America?

It's just another way of abbreviating mathematics. Math is a straight up truncation of the word whilst maths follows the "rules" that a plural is marked in the abbreviation, like nos is an abbreviation for "numbers" (no = singular), even if we now think of mathematics in the singular. They're just conventions that got standardised differently. Both versions have a long history.

I have no idea