Back in the mid-1990s, we were trying hard just to get HTML pages to display properly. About the only dynamic pages were e-commerce sites. Everybody was trying to get e-commerce sites going with Perl, Cold Fusion or whatever else they could find. By the late 1990s, other languages better suited to the web were coming out, like PHP and people started figuring out that it was a good thing for site owners to be able to edit content on their own sites. People started writing content-management systems (CMSs). This allowed people to upload photos, write stories, and made web pages much more interesting. In those days, everyone wrote their own. This was the dawn of the custom CMS. Then some people started commercializing their CMSs and building businesses that sold and supported CMSs. Several of the web-based CMSs were outgrowths of another category of software: Document Management Systems. These kept up with word-processing documents, spreadsheets, presentations: typical desktop-software documents. These were coming to the fore around the year 2000. They were used by large magazines and newspapers, and generally ran to a six-figure implementation budget. In the early 2000s work began on open-source CMSs: Mambo, Drupal, and others. For the first few years, they were only marginally useful, but by about 2004, they were starting to be ready for prime-time. That's the history of CMSs: Custom (c 1997), Proprietary (c. 2000) and open-source (c. 2004).
I'll make the case that there is absolutely no reason to start a project on a from-scratch custom CMS these days. The last site I knew of that went down that path were left high and dry after nine months when the company decided they didn't want to support that software any more. Are proprietary CMSs worth while? It depends on the feature set needed. I'd say for over 90% of the sites out there, an open-source CMS will do the job nicely. Unfortunately, this leaves only a niche market for proprietary CMSs.
I wish I were graphical enough to draw the overlapping bell curves of:
Custom CMS -> Proprietary CMS -> Open Source CMS
See Wikipedia's list of various CMSs