Why do you have to send an edited version of your manuscript to a literary agent when they are going to hire an editor to edit it later anyways?

A real, legitimate agent is

—Not going to hire an editor;

—Not interested in working with someone who cannot write manuscripts that are basically ready to go.

If you are a novelist, the tool you have at your disposal is the English language.

If you can’t use the English language effectively, saying someone else can fix up your prose later is like a carpenter declaring someone else can fix the shed that fell down. Why should anybody hire someone to build a shed when that person can’t drive a nail? Lots of other carpenters are available who are good at driving nails.

Why should any agent work with someone who can’t write effective prose? Lots of other writers are available who can write effective prose. Even these days, when a thousand people a day drop something onto Amazon, agents get hundreds of queries per month.

You also just misunderstand what an agent does. An agent may do some editing for her clients — my agent is a good editor — but lots of agents don’t do any editing for their clients at all. That isn’t part of the essential job of an agent. An agent sends your manuscript to acquiring editors with little enthusiastic notes about why the editor should take your manuscript to the marketing department and try to get them on board about your book. if the acquiring editor gets a green light from marketing and the publisher makes you an offer, your agent goes over the proposed contract and fixes all the terrible boilerplate clauses and argues that you should keep audio rights or whatever else. That is the main part of your agent’s job.

If your manuscript is placed with a publisher, then an editor at that publisher, plus a copy editor, plus a proofreader, will go over your manuscript and request changes. The agent does not hire any of these people. Neither do you. They work for the publisher.