There are a couple of reasons for this. I mean, one, we're frequently protecting them from total strangers going "will you introduce me to your agent?" (My answer, by the way, is generally not a positive one, and on the rare occasions where I offer to introduce someone to The Agent, she gets an email from me warning of the pending introduction. So emailing her cold, saying I sent you, doesn't actually work.) Now, we're not trying to be nasty, and we're not trying to say "no, you can't join our special club." But we're also not going to sneak people we don't know through the back door, past the rest of the pending submissions. We are not magical doors to representation, and because our agents will often feel obligated to look at any real referrals, we try very hard not to be too extravagant with who we send their way.
Two, our agents are the people who take care of us. I mean, as protective as most authors are of their agents, our agents are ten times moreso. They're the ones who understand the weird little clauses in our contracts, tell us when interesting opportunities open up, and keep us from being eaten alive by our editors. When there's a problem, the agent fixes it. (That goes for problems on both sides. If I get behind on my word count, The Agent will probably be poking me about my deadline before either The Editor or The Other Editor. Because that's her job.) Since our agents take care of us, we feel a little obligated to do the same for them, at least to the best of our abilities.
Does this mean we don't want to share our agents? Hell, no. I want The Agent to sign twenty people who become New York Times best-selling authors and get six figure contracts and can afford their own diamond-plated ponies, because then she'd be able to eventually afford an island, and she'd probably let me genetically engineer dinosaurs as long as I kept meeting my deadlines. An agent is only as successful as their client list, and the more really successful clients an agent has, the more not-so-successful but-oh-so-wonderful clients they can afford to keep working with. It turns into a delicate balancing act that I'm really glad I don't have to perform.
Somebody said recently that we don't work for our agents, they work for us, and so the submissions process shouldn't seem so much like a job interview. I view it as more like the relationship between a householder and their butler. A good butler is for life, and that's not a contract to be entered into lightly. If the butler can't like you, then they can't work for you, and you wouldn't want them to. My agent works for me. I also work for my agent, just like most people would agree that Bruce Wayne works for Alfred as much as Alfred works for Bruce Wayne. And because they have that relationship, he gets to go out every night and be Batman.
Why do I appreciate my personal superhero?
Because she lets me be a superhero, too.