That's not allowed because parsing that kind of nesting requires a context sensitive parser, which is much more expensive computationally than context free parsers.
I understand that, you understand that, and your parent commenter probably understands that, but the point is that a user of HTML should not have to understand it.
IRC formatting codes (not standard, but popularized by mIRC and thus implemented by most clients) are also toggles instead of begin-end pairs, so they allow this. If I remember correctly, old text processors like WordStar also implemented formatting as toggles.