They have their own recruiting team that you are going to be dealing with anyway, and aren't playing the numbers game or shuffling their candidates between many different companies trying to get their (usually very large) hire bonus. They just throw as many people they can at as many companies they can till they get a bite.
Senior employees can be up to a FULL YEAR of the hired employees salary. As that employee you don't pay it, but it's not something that is benefiting you either. On top of that, the companies often have claw backs. If the employee leaves under a certain time, they have to give back their fee.
The reason they don't respond after the first email, is that they need someone who is going to be very proactive and actually motivated to get hired. Else their candidate catapult might hit the target, but less likely to stick.