The css contains form elements you might not be using on that form because the same .css file could be used for all forms in your site, so just because you don't have a datepicker in this form you still might need it in the css.
Also we allow you to edit the form contents and design (css) separately. So you could add a datepicker to your form at a later date without updating the css, so the css has to be prepared for that.
We do appreciate the feedback and we will be investigating ways to simplify the html and css in future updates.