Testing for what? That the card hasn't been cancelled or has zero money on it? Can they test for how much money is on the card or anything else useful?
And they need bots because they might have say 1000s of cards from a database hack and most of the cards won't be useful?
What kind of large dollar purchases would someone try to make once a card has been confirmed? Why not let bots attempt lots of large dollar purchases?
These brokers don't want to get involved in any significant fraudulent charges for various reasons.