Despite what it may seem like, I'm really not trying to mis-parse the reasons here, I'm honestly trying to figure out where the line is, and why it's there.
The screenplay is still being written in letters. Mathematical ℝ is more accurately thought of as an ideogram than a letter. If you were to write "let r be a member of ℝ", the "ℝ" would be structurally parallel to the full word "member", not to the "r" within it.
Courier for screenplays is a choice you make at the document level; blackboard bold for mathematical entities is not. ℝ is always ℝ no matter what styles apply to your document.
Though, as I understand it, the full-width characters are there not for any modern use cases, but for historical reasons having to deal with older character sets.
Still interesting.
A clean grid would be desirable in formal use, but formal use means trying to avoid Latin characters as much as possible. It's generally possible. Plaques and the like are much more likely to say e.g. 二〇二〇年 than to say 2020年.
And I doubt you'd want to eliminate all formal uses of Latin characters. E.g. a plaque about a person would likely want to use their preferred name, which might be in Latin characters.
V A P O R W A V E
ＡＥＳＴＨＥＴＩＣ
But, note that the identical process, much earlier, is how we got separate capital and lowercase forms. Writing systems never do that when they're developed.