When receiving unsoliciting phone calls by telemarketers, many people consistently hung up, don't bait, and don't interact. So why don't telemarketers delete from their databases such phone numbers that don't lead to any sales or other business benefits?

Maybe the cost of keeping the numbers is so low telemarketers just don't bother. Or keeping track of what numbers to delete may actually have a cost. Or perhaps telemarketers hope those people will eventually pick up the calls.

Any insight?