I've played around with WordPress here and there for a few years and am wondering about problems with CSS validation with WordPress themes. This may seems like a stupid question, but:

Is there a reason, relating to WordPress, why it appears no themes I've run across come up error-free in WC3 CSS Validation?

I thought of a number of reason this may be happening:
-Those designing many themes not caring whether or not it pass validation
-I'm getting my themes from not so great sources
-There are quirks with WordPress that force designers to "hack" at their CSS

Just wondering.