This happens in countless movies, TV shows, and games. If the hero, or villain, would just kill the other when they get the chance, like they probably would in reality, they'd win on the spot.

Instead they have to let the villain monologe, or finish their plan or let them live every single time. Villains let the hero give some speech or put them in an elaborate death trap rather than simply killing them.

In conclusion, characters are morons.