WordPress tends to add a LOT of “stuff” in both the final HTML, and all the layers that build it. This is done to accommodate the masses. As a developer you can write <img src=’images/image.gif’ alt=’test’> or you can ping a database, resize an image, pull the proper alt tag, construct the final img tag then output it to the screen.
All the same, WP adds code to your header to account for all the functionality it MIGHT use. Not necessarily all the functionality it does use.