Molly, if they handed out training schedules that included, say, a long run of 23 miles, you'd get to 23 miles and then say, "Goddammit, I might as well do the 26, plus the 385 yards, seeing as I've come this far!!"
Then, as you'd already done the marathon you'd been training for, you might not bother turning up on the big day itself.
So it's too risky including a run longer than 20 miles in the training schedule. (But that doesn't stop you going further if you want to, of course. They can't stop you.)