Why are so many books utterly and totally lacking in physical descriptions of characters?

Bonus points if the author first mentions a specific trait, physical build, or whatever else halfway through the novel and totally fucks up my mental image.

I don't expect a biology model description for each character, but write me something brief and evocative of how they should look, you fucking dork author. I don't even know how tall she should be, her hair colour, anything. Why are you like this, author?