The gist is that the agency is going to demand a fee for bringing you in. A lot of times it’s a percentage of your offer. Thus, you get a smaller offer to offset or reduce the cost.
I am conscious of which roles I offer to outside agencies, but once I do, I stop worrying about the cost and focus only on the value I can get from the candidate. Those agency roles are some of the most competitive positions; I don’t expect to be able to hire the best who happen to not realize that $0.75x is smaller than $1.00x for positive values of x.