Because I don't want that cognitive overhead of even thinking about what the selector should be. Then what if I have another sub-sub text? Do I make that an h3? An h4? I don't want to have to think about that. I just make it "text-xs" and move on.
I don't even need to move over to my css file at all - I stay within my template and stay within my flow.
It seems crazy and counter-intuitive at first, but I find that it makes me a lot more productive.