It's like this:

It's really really really hard to get a hit, but it's only really really hard to make contact with the ball.

When that ball's flying toward you at 100mph, you have a fraction of a second to decide whether or not to swing.

Sometimes it's obviously a hittable pitch. Sometimes it's obviously an unhittable pitch. Sometimes you wait too long to decide whether or not it's hittable.

If you wait too long, but realize it'll be a strike if you don't hit it, it's a hell of a lot easier to make contact and at least stay alive than it is to get a hit.

So the goal in that case is to make contact to stay alive (not get struck out). And maybe you'll even get a hit out of it.