Building good websites is not as easy as it appears at first sight. Then you want to add advanced features, that allow readers to contribute, to communicate, or to buy things. Suddenly that simple site needs to be up 24x7 hours, people have questions, need support. If you were running a small company, creating a web-site that allows people to buy things means that you are open to the entire world. Now your simple web-site, created by perhaps a graphics design company or your PR agency, needs to be available all the time.
Many vendors claim to have tools.
Yet all these tools are worthless if you or your supplier do not have a good, solid background in engineering reliable systems . Then there are competitive pressures. You cannot afford to develop it from scratch. You have to buy products. So you buy ShoppingCart-2.3, the latest and greatest this week.