It's exciting and unnerving to send out a manuscript, perhaps more so when you know what you're actually doing. The first time, we may think that the MS is perfect, the agent/editor will gasp and say, "This is exactly what I've been searching for," and we will be on the way to reader adulation. By the tenth or hundredth time, you've done some research and you know a few things.
First, it probably isn't perfect. No matter how many times you check, you'll probably miss something that you should have caught and corrected.
Second, there is the skinniest of chances that an agent/editor will ever see it. In all probability, some sweet young thing who's trying to become an agent, editor, or famous writer will be on duty, sorting and sifting with a list of reasons why she may throw your MS in the trash. No title page? Gone. Margins not perfect? Toss it. Query that's ingenuous, boastful, shy, tired, too ordinary or too extraordinary? Don't need it. There are thirty more in today's stack.
Third, if an agent or editor does get your MS and if she does say, "This isn't bad," that's one hurdle. From there it goes to a whole bunch more editors, marketers, and kingpins who make a final decision. "No. We don't need a mystery right now." "No, it isn't what the public is buying this year." "No, we've got something similar by a bigger name, so we'll go with that." No.
Even if all those hurdles didn't matter, sending in your MS is still a commitment on your part. You're saying that this is the best you can do, and it's probably the only chance you'll get to say it. Sure, if it's accepted you'll get chances to tweak here and there, but putting that postage on that box or hitting "SEND" on your email says, "I've done what I can with this one."
And that's why we do it, whether it's print and package or file and attach. What we can do isn't enough. We want to know that someone else gets it, approves of it, maybe even stays up nights to finish it. And even the overwhelming chance of rejection can't stop us from trying.