The reason is that in a web site the result of your labor appears differently in almost every monitor that represents your creations. Monitors are calibrated at different resolutions. Monitor resolution can dramatically affect the coarseness and detail of graphics and even text. Monitors are different sizes. Web sites have to take into consideration that very small monitors have to be able to follow what you are communicating.
Monitors have their color calibrated differently.
Someone may be viewing your web site with several pieces of software running at once. This often dramatically affects the reproduction of a web site's graphics. This, and like considerations, makes it useful for web graphics to be high contrast with clean lines. People running Windows view the graphics on a web site significantly darker than someone with other operation software. In addition, all text will appear larger, bumping blocks of copy into different configurations than someone without Windows.