1. Things get lost in the post, piled up in heaps, slip down the back of desks...You could be waiting indefinitely. Ten years later and I'm still waiting to hear back from one agent. (You're too late! Ha!)
2. Assuming your ms has reached the agent, you could be at the bottom of a very big pile of unsolicited submissions. The agent has to prioritise their existing clients so it's understandable that at times the slush pile is left to mature quietly on its own. It can take months for an agent to read your ms, only to immediately discard it.
3. Have you ended up with your first boyfriend/girlfriend? Yes, some of us get lucky and never look beyond that first snog at the school disco, but for most it takes time to find our match. Similarly you want the agent who not just likes your novel but completely falls in love with it. The chances of finding The One on the first submission are as likely as on the first date.
4. Most agents don't send back messages such as 'This novel is an abomination' if they don't like it, but there are a few who don't believe in holding back their opinion. I have been at the receiving end of a letter from an agent that confidently stated 'You are wasting your time.' Luckily I'd already had 2 positive responses from other agents so it didn't matter too much, but I think if that was my only reply I might have given up there and then.
5. Do the maths. If you send out to one agent at a time and each takes 6-8 weeks to reply, it's going to take 1-2 years before you've had a chance to send out to 10 agents. If you're sending out to 6 each time, then you'll have clocked up 40-50 in a year. (Gosh, that sounds depressing but...)
6. Novels are of the moment. You've written it, re-written it, edited it now. If it does the rounds for several years it will be slightly out of date, perhaps a bit tarnished round the edges. Better to get it out there as quickly as possible and see how it fares in the marketplace.
7. A positive response is to be cherished, even when it's a no. You examine the comments the agent has made, wondering if you should apply their suggestions and start a re-write. This can drive you potty. Multiple submissions should give you several responses to consider. If all of them are telling you the characters are weak, then you can be pretty certain that you need to look at them again. If one says it's too long, but another says it's too short, you can make your own decision. It's all opinion, but the wisdom of crowds means more opinions are likely to give the right answer than a single opinion on its own.
**** Please note, I'm talking about the initial submission ie 3 chapters, covering letter, synopsis. It doesn't take much time for an agent to look at them and make a decision if they'd like to see more or not. However, reading a whole novel with a view to representation is a completely different thing. If an agent asks for the whole book you shouldn't send it out to anyone else for several weeks, and then give the first agent notice that that's what you intend to do.