Why is a kitchen counter called a counter?

That's easy, see this definition on Wiktionary:

https://en.wiktionary.org/wiki/counter

Quote:3. A table or board on which money is counted and over which business is transacted; a shop tabletop on which goods are examined, weighed or measured.

Over time it's expanded to mean your kitchen benchtop, even if you don't count or weigh anything on it. But maybe you should - try some baking!