Yep it seems baffling, especially on sites added in the last 10-15 years.
Tens (hundreds) of thousands of dollars go into the process with the creation of nomination files etc, you would have thought a one or two page website would at least be a by-product.
Sounds like a good business opportunity if anyone were so inclined!
Actually I've found it quite interesting while travelling around constantly, seeing how different countries approach their sites. Even countries you might consider as "culturally similar" have vastly different approaches to websites, signage, brochures, tourist info etc. We've visited sites in probably 35 countries over the past 18 months and the differences really stand out sharply. EG in Czech Republic there's a lot of focus these days from official channels about their sites (part of the strategy to get their visitors going to places other than Prague!), but next door in Slovakia there's almost nothing.