Your Other Teeth Start Shifting Into the Gap
Teeth are held in position partly by the steady pressure of the teeth around them, pressing against each other in a kind of balance across your whole mouth. Remove one, and neighboring teeth can gradually tilt or drift toward the empty space, while the opposing tooth above or below the gap can start to over-erupt into it since there's nothing left to stop it. Once teeth have shifted, straightening them out again usually takes more time and treatment, sometimes orthodontics, than it would have taken to simply fill the original gap when it first appeared.