answers to the following questions
1. Because the airlines try to set up a situation in which each agency tries to undercut the rest, and ask several to supply for the same job. This usually guarantees the worst service from the agency/ies that get the job, but the bean-counters go home proud of their work.
2. The agency can only recruit to the specification provided by its customer, while doing its polite best to get the customer to provide one at all, apart from "gissa Airbus/Boeing pilot".
3. If the agencies were not there, you would have to spend your day trying to contact directly the people you think may need your services; if and when this succeeds you're on your own with tax, terms, insurance, etc etc if you are a contractor, to say nothing of the fact that the agency must pay you promptly while an airline that employs you directly feels no such obligation.
There are good agencies, bad agencies and crooked agencies. Your skill is identifying a few good ones that their airline clients trust, and doing all you can to make it easy for them to sell your services. And yes, the good ones do earn every penny of the mark-up. The bad ones don't. A good agency will have large cash reserves against the fact that they pay contractors on time while their customers take 2 months if they don't collapse outright. A bad agency can only pay you when their customer pays.