Because opportunities for doing so are few and far between.
When you were learning to shoot a bow, you discovered that on occasion, if the two deer broke for cover at just the right angle, you could nail them both, two arrows but one shot. Later, as you became an adventurer, you discovered that the technique worked just as well in combat; the moments when the enemies were moving at just the right angle were few and far between, but when they did, bam! You were ready to take advantage of it.
In other words, some of the powers allow the player to take a tiny bit of narrative control. You're not just deciding "my character is using his power," but also "my character has spotted the opportunity to do so."
Thus, it's a combination of ability and opportunity both. Sometimes you have one, sometimes you have the other, but only rarely--in game terms, once a day, for ease of play--do they both align.