I think half of the answer is in your post - they leave it more unguarded, so it's a great target and struck easier and more frequently. On a casual observation level, I've noticed most people keep their sword pointed upright at least at a partial angle, instead of downwards in pflug / alber (I think that's the name of the stance). That means blocking a leg shot is the longest possible travel their blade needs to make to successfully block, and means they have less time to react to block succcessfully, reducing the chance for them to block.
I started with eastern martial arts and I've always had the habit of "leading" with my leg and then stepping in for follow-through strikes - when I took the 1 class with John Clements of ARMA, he said the proper posture is leaning forwards with your body, at or in front of your leading leg. That makes your leg less of a target, and gives you greater measure for striking. Personally, it was an uncomfortable and unnatural position for me to fight from, as I'm more of a counter-attacker than an offensive attacker, and the "partially retreated" stance gives me more time to react and a better "view" of what my opponent is doing - especially the legs. For the more offensive people, it's a more suitable stance and it also reduces the legs as a valid target area - but at the expense of presenting more body as a target.