You only have to define it once in a document, book, whatever. Also, it’s not like you’d ever need to do this for handwritten notes, only for a wider audience, or if you intend for something to be read by not just you.
No one is suggesting you don’t use symbols, just that you define them, and not assume the reader uses the same symbols as you. Which, so often, they don’t. (How many different ones have you come across just in highschool and uni. I came across multiple)
I’m no physicist, but surely there is a huge range of symbols for the same thing, especially the more niche you get.