Because a catcher must catch a third strike for the at bat to be recorded a strike out. Otherwise the runner can advance to first
Uncaught third strike
The rules of baseball differ slightly from league to league, but in general share the same basic gameplay.
There are several major rules codes, which differ only slightly.
In baseball and softball, an uncaught third strike (sometimes referred to as dropped third strike) occurs when the catcher fails to cleanly catch a pitch for the third strike. A pitch is considered uncaught if the ball touches the ground before being caught (a bouncing ball), or if the ball is dropped after being grasped (see also catch). In Major League Baseball, the specific rules concerning the uncaught third strike are addressed in Rules 6.05 and 6.09 of the Official Baseball Rules.
On an uncaught third strike with no runner on first base or with two outs, the batter immediately becomes a runner. The strike is called, but the umpire does not call the batter out. The umpire may also actively signal that there is "no catch" of the pitch. The batter may then attempt to reach first base and must be tagged or thrown out. With two outs and the bases loaded, the catcher who fails to catch the third strike may, upon picking up the ball, step on home plate for a force-out or make a throw to any other fielder.
First base, or 1B, is the first of four stations on a baseball diamond which must be touched in succession by a baserunner in order to score a run for that player's team. A first baseman is the player on the team playing defense who fields the area nearest first base, and is responsible for the majority of plays made at that base.
In the numbering system used to record defensive plays, the first baseman is assigned the number 3.
In baseball, a foul tip is defined as "a batted ball that goes sharp directly from the bat to the catcher’s hands and is legally caught. A foul tip is considered a strike and the ball remains "in play."
A foul tip is not the same as a foul ball, although many people mistakenly use the term to refer to any pitch at which the batter swings and makes slight contact, regardless of whether it is caught by the catcher. However, the rules are very narrow: it is not a foul tip if the ball touches anything else on the way to the catcher's hand or glove or if it is not legally caught and held. Any thing else is technically a foul ball, including if the ball is caught after popping up into foul territory.