> It makes no sense to replace a wider sentence space with two regular spaces.
Using a double space is a simple way of telling the computer that you want a wider space, because otherwise it can't tell that you mean the end of a sentence.
Three hyphens is a lot. I would have said it was more common to just use one hyphen for what should properly be an en-dash (which most people don't use) and two hyphens for an em-dash with or without spaces to separate from the surrounding words.
I can't recall ever seeing three hyphens used to signify an em dash in a document published on the net. (I added the word "published" to account for the possibility of the existence of a markup language I am unaware of that employs your three-hyphen convention.) I am fastidious enough about usage that I probably would've noticed and remembered.
In my experience, its always two hyphens (or space, hyphen, hyphen, space).
Using a double space is a simple way of telling the computer that you want a wider space, because otherwise it can't tell that you mean the end of a sentence.