Why is this category called Science "and" Mathematics?

I agree that the name is a misnomer. Mathematics is certainly a science even if it is a formal science while nearly all subcategories of "Science & Mathematics" are natural sciences (with the exception of Mathematics, Engineering and the so-called "Alternative" subcategory. Well, Agriculture is also a kind of engineering, IMO)

On the other hand, social sciences exist in a category of their own which includes sociology, economics and.. dream interpretation. Computer science, by the way, does not seem to exist even in Computers & Internet.

If it were up to me, I would have renamed this category to "Science and Engineering", and I would have moved "Alternative science" away, perhaps grouping it with other New Age stuff somewhere under Society and Culture.

because under mathematics you have things like geometry, and trigonometry. In science, you have biology and chemistry.

Mathematics only really falls into science when it comes to physics.

While math is frequently used in science, it is no more a science than Sanskrit is. You can not formulate and test hypotheses in math the way you can in science. Math is lumped in with science because there is no better category for it. Logically, it is more like a language but the people interested in linguistics are less likely to be interested in math than people interested in the sciences would be.

Other answers are wrong. Maths IS a science, it is the science of numbers, shapes, and other things you learn about.

Science is about testing hypotheses and proving that things are true. Maths is the same.

Mathematics is abstract. You use it to prove scientific principles. But, it's not a science in itself, per se. It's more of a discipline, like music.