Arguably it's because websites (or more to the point web applications) are becoming more complex
In my experience, the widely-used ones tend to be fairly sane for the most part, and can often be used without having to delve too much into the complexity unless you need to - which I think is the important bit
In my experience, the widely-used ones tend to be fairly sane for the most part, and can often be used without having to delve too much into the complexity unless you need to - which I think is the important bit