Well you should be able to style them differently. Problem is, outside of basic text fields, textareas and buttons (which can be styled completely through standard CSS), many form elements like select menus, sliders, radio buttons, checkboxes and others can only be styled through browser specific prefixed code, ugly hacks and the not quite official and not quite obsolete appearance property.

If all these fields had nice clean ways to style them so they looked the same cross browser (even with different defaults and behaviour), then there would be less people using these giant scripts and frameworks.