Why use a maximum line length at all?

There are many discussions about what the maximum line length should be (80, 100, 132 are all popular).

But this all assumes there needs to be a maximum line length at all.

I can see why many people don't like horizontal scrolling. But why not let the editor do the line wrapping?

I can see two options:

• Lines should be wrapped at specific points to be at all readable: this would probably need some improvements in common editors, but I'm sure that if the rules for readable wrapping are that specific, they could be expressed in code.
• It doesn't matter much where lines are wrapped: that's really easy then.

The advantage of letting the editor do the wrapping seems obvious: people can use whatever line length they like (even switching to shorter lines in diff-mode or something).