Some are outstanding, and some, well, need more practice. Publishers feel inundated with unsought manuscripts, and frankly, most are never read. The best agents work hard to establish a reputation for excellence, because the rest of the agents simply cannot get many of their submissions read. So, the best agents are the ones who sell the books. You want one of those agents representing you, and your challenge is to attract one.
Publishers have quotas in various genres.
When a quota is filled, they won't look at books in that category, period. If your book is quite similar to another book a publisher is developing, even if yours is far better, it won't be considered. A publisher may feel that a particular genre is on the way out of favor with the public, lucky you. When a publisher is interested in your work, the agent will forward part or all of your manuscript .for scrutiny. If the publisher likes your writing, the editors will want to see more.