This could have been one long post, but I think three posts (here and here as well) is better.
So, the big reason for the lack of a decent (but lets not confuse that with uninformative) website is governance.
Like most Washington state counties that have not gone through the home rule process, Thurston County is a collection of independent elected officials from three county commissioners to a sheriff and an auditor and more. In the current way things work, each independent office has the job of maintaining their own website. So, what you get is a mishmash.
My understanding of how things are run right now is that a department under the commissioners, central services, provides general web support to most of the county. The independent offices, auditor, sheriff, and the rest, get some technical support, but tend to maintain their sites on their own. Most of the folks that develop and maintain the websites for the independent offices do so on a part time and ad hoc basis. And, when a person who then leaves the county who was maintaining of the sites, but didn't have that job officially part of their job title, they take that experience away with them.
So, instead of a fully funded web administrator at the middle of all the offices, you have part time web managers throughout the county, each with their own ideas and levels of experience. That they the sites look different doesn't matter to me, but the redundancy in duties and differing ways the sites are put together matters a lot.
I think the county would actually save money and be able to develop a better overall web presence if they rolled all the web functions into central services. Not saying that the folks in the independent offices who maintain the websites don't work hard, just saying they work a bit too hard.