Much of client side web technology today runs directly from source, E.g. HTML/CSS/JS... , which means source code is readily available to anyone who wants it, a far cry from software where source code is often kept tightly under wraps. As such, it is very simple to just copy out chunks of code and include it in your website.
And since it is just code, it seems to be quite difficult to track compared to stuff like images. Especially when it involves just a few lines of code, e.g. 10 lines of CSS for a button, how do you prove that those 10 lines were copied and not his own code. Because of this, web developers in my opinion tend to be more relaxed about their code. To be honest, I hardly write CSS code for elements such as buttons, borders and such. Mostly because I am unable to get that polished look due to the carefully calculated gradient of the background and subtle changes in color when mouseover. Yes, the designers might have spent hours trying out different color combinations to achieve that perfect result but sometimes, when its just that 30 lines of code, the temptation to just copy might be too much to bear.