They are going by volume, so the overall successes matter and the reason for why the rest are unsuccessful doesn't matter.
Phone numbers get reused all the time, so if they pull the number from the pool they miss a possible future opportunity. This is important when lack of success would massively shrink their pool of numbers at no real cost savings to them since they are going for volume anyway.
Basically you are asking from a logical and well intended point of view, but telemarketers are approaching it from a maliciously logical volume method that benefits from stumbling across enough gullible people to make the rest of the volume worth it.