The main reason is that you will be able to pick up a humanities/social science course with the skills you already have at the end of high school. What do people do in those degrees? Read text, think critically, express ideas. And you've not only practised that a fair bit in school, but also after education, where you're reading the newspaper, looking at art, and so on. Importantly you aren't going to lose the text reading skill because you'll be using it a little each day.
So you have plenty of practice in the humanities, at least to get you started doing it seriously if you need it later in life. And you have enough that you're not gonna be surprised when someone mentions there was a civil war in the US.
Math stuff seems to be use it or lose it. You can easily avoid linear algebra and big-o thinking, and you will if you aren't forced to think about it. If you do a STEM degree at least you will have heard of stuff and practiced a load of otherwise obscure topics, giving you a starting point that isn't years behind on prerequisites, and leaving you where you won't know what's interesting.
I've met plenty of STEM people who knew a thing or two about history, economics, and the arts. It's relatively rare the other way round, though you do get the occasional renaissance man from that side.