Because the hero wouldn't have any one/thing to rescue. Recommend you watch The Expanse, that has drones.

I always thought most battles on Star Trek could've been solved by using the transporter to place bombs or killer robots on the enemy's ship.

Today's technology, battle ships don't even need visual contact to be in range. Any space battle soul be between ships kilometers away.
But good sci-fi will always give some excuse to avoid automatization.

It would ruin the plot it's like in scary movies when everyone has to separate instead of staying in the group where they might have a chance to survive

They always have vents people can escape down though