Lots of things.

1. They're just generally marketing ploys

2. I have more than enough email, and I never read anything unless it's from someone I know.

3. I prefer information where everybody can see it - that way there is no manipulation involved.

There is no reason that these sites can't simply allow one to sign in and see what is available. When they have to send you an email, it's because they want to sell you something.