it's not so much remembering what ∑ means insomuch as that it's completely impossible to google the first time you run across it. It'll be in some PDF that doesn't allow you to copy-paste the symbol and you won't know what it's called. Rinse and repeat for any of the million symbols mathematicians use, never mind that loads of symbols are context dependent even if you could google them.

I hope mathematicians have a better reason than "it's tradition" for making the entire field completely opaque to anyone who hasn't studied math extensively.