Because your knees aren't designed to take those sorts of forces. Yes, we will all find ourselves having to grind up hills, or pushed into a nasty headwind, but if you've started with too high a gear at the bottom, you're just making everything worse.

One of the skills of riding fg is riding softly, ie, despite being forced into a low cadence, you aren't driving high forces through your knees - funny concept but you'll know it when you experience it. Having sufficient leg strength helps of course.

The point I was making is that riding fg does not equal stuffed knees. You see riders on the latest uber racer mashing away with a lousy technique and a low cadence - they're doing as much damage as anyone else. For fg, select your gearing based on your body and the roads you ride, NOT hipster fashion or what some idiot on the internet told you.

Note: I am not an idiot - insane, delusional and deeply disturbed, but not an idiot... my cat told me so.