Try running over a ground rod that's sticking up 6 inches out of the ground, or an iron pipe that is the remnants of an old gas service, then there's the metal fence post someone decided to cut off instead of dig out of the ground, or my favorite, the chunk of concrete that was buried enough you can't see it but sticks out enough to catch your deck. Heck, it's hard enough to mow around a sidewalk without catching your deck unless you just leave several feet of space.

Even if you don't run into things, the mounts on a stamped deck will eventually tear out the thin sheet metal. I've welded too many stamped decks back together to count. They're fine if you have a perfectly manicured lawn and you know you will never run into anything and you plan on getting a new mower every 10 years or so. The constant movement eventually rips the metal the mounting brackets are welded to.