Also former infantry: To expand the point, in US Army doctrine, an obstacle has one of four effects, block, turn, fix, or disrupt.

Typically, you use obstacles in engagement area development as part of a defense. An obstacle that's not observed isn't valuable because it's easy to circumvent. You can probably use a bulldozer (guessing, I was light infantry, not a tanker) to breach the dragon's teeth relatively quickly so the obstacle doesn't have the intended effect. However, it's much harder to breach the dragon's teeth if someone is shooting at you. Now you need an armored bulldozer, or enough suppressing fire to cover the breach.