grammaticality - Why must the future tense not be used after 'unless'?

[Source:] !! Do not use the future tense after unless
[✓] I won't go unless you go

[✘] [I won't go] unless you will go. [✘]

Why not? Even if the above is prescriptive, what may be some reasons? In my separate question here, I quote: unless = if not. If not can precede future tense; so why can't unless?

Footnote: This Wordreference.com question motivated this.