My manuscripts are mine.
When you publish something, you generally lose all control over who reads it. Anyone who's willing to shell out the money -- or anyone who has a library or a friend willing to shell out the money -- can read what you wrote. It is public. It is a matter of record. It is no longer completely yours.
Unpublished manuscripts are not like that.
You don't have some divine right to read one of my unpublished manuscripts if I don't want you to. I don't care who you are or what our relationship is -- friendly, romantic, familial, whatever. I don't have to give you a copy in any particular stage unless I want to. In these early stages, I may give you a copy to read just for fun if I'm feeling magnanimous, but I don't have to. If I ask people to read and critique my drafts, that is a fairly selfish act: it is for my benefit in improving the novel. Their potential benefit in enjoying the novel (or, hey, short story, essay, whatever) is not the main thing.
I am not obliged to care what you think of my writing.
The obvious example here is my grandmother. You all know I love my grandmother and have a very close relationship with her. But my grandmother reads biographies and Christian historical romances. Asking her for input on Thermionic Night and rewriting it to her standards would be stupid, and we both know that, because she is not my target audience. Readers "like her" will not generally be readers of my books, unless you count the great-aunts. But they'll read whatever I manage to get published no matter what it's like, so rewriting for them is also silly. (Some people write gateway speculative fiction, SF or fantasy or the like for people who thought they didn't like it before. scalzi has talked about doing this with his books, and I appreciate it. It's a good thing for someone to be doing. It is not the only thing for anyone to ever do, and it's not what I'm doing with Thermionic Night.)
There are subtler cases -- if you don't ever like aliens in SF, ever ever ever no matter what, I'm not going to ask you to alpha read a first-contact novel. But I'm amazed at how many people don't get the cases I consider obvious.
Even if I care what you think of my writing, I am not obliged to ask your opinion at any specific point in the process.
I only had four alpha readers for Thermionic Night. I am interested in the opinions of more people than that. I value the opinions of more people than that. But there is only so much data I can process at various stages in the writing of a book. Having four people who read a good deal in the relevant genre, who in some cases write and publish in it, and who have a good mix of knowledge to catch my infelicities and ignorance to catch when I'm talking past the audience, was for my benefit and the benefit of the book. I will have more than four people looking at the book in this next draft. But even if I love you, and even if I respect your opinion about books, nothing says you have to be one of them.
I am not on a humanitarian mission here. I am not providing book relief to the bookless. I am trying to write a better book. If I'm trying to be nice to you, I will bake you bread or give you a hug or something. Reading and critiquing my book is a favor you are doing me, not a favor I am doing you, and you can never, ever demand that you have to do someone a favor they don't want. I can choose to give you a copy just for the fun of it, but I don't ever have to. Writer brains sometimes function weirdly, and if it's going to interfere with a better book later to give you a draft now, I'm just not going to do it. Period.
Pitching a fit and behaving as though you have been sorely abused because you do not yet have a copy of my latest draft is not a good way to convince me that I should share unpublished work with you. In the future, if you are my agent or my editor and I'm under contract and wibbling, then yes, I have signed you on for specific rights that may include haranguing me to send you the book. Until and unless? No whining, no fit-pitching. Behave like a grown-up.
I don't feel like I should have to say that, but apparently I do.