Why doesn’t the team with the hammer just knock out all the other team’s stones every end and then score one in the 10th?
Ah, now you’re thinking like they did in the 1980s. That was the strategy employed back in the day and made for some uninteresting and predictable curling games.
Things changed in the 1990s with the free-guard zone and the four-rock rule. The free-guard zone is the arena outside the house from the tee line (the horizontal line cutting through the middle of the house) up to the nearest hog line (the horizontal line where rocks must cross in order to stay in play).
Stones sitting in this area are called guards and cannot be removed from play until four stones have been thrown (aka the four-rock rule). This allows teams to place guards and then draw around them with their following stones so that they’re harder to eliminate.
The fifth rock of play is the first one that can eliminate guards.
There are still ways around it as teams can tick guards — but not eliminate them — so that they’re less troublesome.
Additionally, teams with the rock are generally looking to score more than a single point.