The main reason is they are afraid of getting viruses through attachments, so they don't want you to send anything attached to the email at first. Some get around this by asking you to include the sample in the email body, as Anya mentioned, and others have a private upload system that protects them from viruses and allows for a low labour - automated - process. Either way, they don't want to take the risk unless they know they are interested in what they are reading and can trust the sender more than somebody who just spammed them their manuscript.
I don't blame them, the internet world is getting to be an increasingly dodgy place.
Which is all well and good, and I sympathise, but they don't need to tie it down to a single page and have an expectation of a formula that in no way reflects the book, or the kind of writer we are.
The Hiassen and Pratchetts of this world are suited to sound bite, their style is snappy, it reflects their books.
I am selling mine on a query that absolutely doesn't reflect the style, or feel of the book, because it's snappy. Which means when they get to it, and realise the MC isn't a gung ho super hero (and that's the point of him, he isn't supposed to, he's supposed to be the antithesis of the super hero trope), they're going to put it down and say, hey this isn't what she sold. No, because saying in thirty words that I have a dreamy dillis of a hero who's going to take a while to shape up - hopefully in an enjoyable fashion - before stepping up to the mark, isn't snappy. Oh, and he's going to have a nervous breakdown and still be the MC... Yawn, move on.
I'm sorry, but I can't agree that a business model that reduces us to a two paragraph holywood trailer, and chooses a book based on that, is the right one.
But, hey, it's the rules, so we play along.
Anyway, Harebrain... need an accomplice?