Firstly, people do. Secondly, those who do are uninformed. Declaring your work to be "public domain" does not actually place it in the public domain. It in fact remains copyrighted.
Passage into public domain occurs when a copyright expires.
That's why Creative Commons has a "public domain declaration", which includes an all-permissive license for if that isn't possible.
By the way, I completely forgot to mention another reason for avoiding a public domain declaration, and going with a license: namely, liability!
The liability disclaimers in, for instance, the BSD license are important.
In fact, I have come up with a slightly modified BSD license.
http://www.kylheku.com/cgit/txr/tree/LICENSE
My version makes it clear that retaining the license is a condition of redistribution, and that use of the software is subject to agreeing with the disclaimers.
The original BSD license lumps redistribution and use together, and only stipulates that the license not be removed.
All proprietary software EULA's I've ever seen make it clear that you accept the liability disclaimers as a condition of using the software. (E.g. by clicking on some Agree button, or opening a shrinkwrap or whatever.)
The disclaimer part of a freeware license is in fact a use license; they are not strictly redistribution licenses.
(By the way, I don't think it's a good idea to go around customizing de facto standard licenses willy nilly; I thought long and hard before making the decision to fix the BSD license for myself.)
In particular, it reflects the fact that I don't think readers of my work should be obliged to follow any rules I might set (license) when they turn to the creation of derived works. They should go and do as they like and, where appropriate, acknowledge prior art (as a matter of professionalism).
I also feel that the problem of associating each "expression" with it's legal creator is a nasty problem fraught with difficulties; certainly, in this regard, you can say that I choose public domain because of its simplicity.
By the way, djb has argued in defence of the public domain here:
http://cr.yp.to/publicdomain.html
Anyway, I'm interested to read other developers views on this topic but I'm no authority and don't want to give the impression that I think it's unreasonable to choose differently. And anyway, most do! GPL, BSD and the rest seem much more popular than public domain.