Why does everybody write their own CMS?

There are two website in my life. First, the one I'm paid to look after as a product manager, this is my company’s, extranet site that provides information on our product and allows customers to log support calls. The second is the ACCU's website, yes, it looks a bit dated, we been trying to put a new one in place using a subcontractor but things haven't gone smoothly as we'd like.

In both cases, the companies concerned have written their own CMS system - that is my employer the subcontractors. When we asked for tenders to develop a new. ACCU website four out of the five submissions used proprietary systems.

Yet the world is full of off-the-shelf CMS systems, for example Plone, Mambo, OpenCMS, Microsoft SharePoint , and the list goes on. Yes, some of these are portals and some provide other functions, but my general point is: There are a lot of you choose from, without writing your own.

Yet, everybody seems to write their own, since getting involved with these websites I have spoken to several companies, and many individuals and it is a reoccurring theme, everybody writes her own CMS.

I think part of it is that CMS systems should be simple, and anyone who has a little bit of development knowledge feels they can write one.

And it seems that many of the CMS systems available are very complicated.

The term is CMS is quite broad, and covers a lot of ground, it seems mean different things to different people.

And add to all this the fact that so many CMS systems are actually evaluating them

I sure there are more reasons, it would seem that nobody has written the perfect, or even the 80% good, CMS system. So maybe there is space on the market for one more CMS system...