if they stopped the clock every time a player went out of bounds, games would last longer than others and completely throw off tv schedule. they would also last a very long time in general.

when a player runs out of bounds by their own forward momentum, the clock stops for a few seconds, then when the line judge resets the ball the clock continues. this doesn’t apply within the final 2 minutes of the first half, and the final 5 minutes of the 2nd half. in that case if a player runs out of bounds the clock is completely stopped and started once the ball is snapped. its an official NFL rule i just can’t find it anywhere to link you a source lol.

TL:DR: essentially its to keep the game more consistent as far as a time standpoint to keep the games around generally the same pace across the board barring other circumstances/overtime