According to Overdone: why are restaurant websites so horifically bad?, here's the answer:
Restaurant sites are the product of restaurant culture. These nightmarish websites were spawned by restaurateurs who mistakenly believe they can control the online world the same way they lord over a restaurant.
Substitute winery for restaurant and you're there. The wine business, in general, is about things being "just so," from the blending of juice to the look of the label. There are a lot of recovering type-A people in the biz.
There are other factors at work, as this quote points out:
"Say you're a designer and you've got to demo a site you've spent two months creating," Bohan explains. "Your client is someone in their 50s who runs a restaurant but is not very in tune with technology. What's going to impress them more: Something with music and moving images, something that looks very fancy to someone who doesn't know about optimizing the Web for consumer use, or if you show them a bare-bones site that just lists all the information? I bet it would be the former—they would think it's great and money well spent."
which I think does a lot to explain why winery websites frequently fail to work well for visitors. And, as the article also points out, many designers get paid by the hour, so it's in their financial interest to create works of art, which may or may not meet the needs of visitors. (Hate mail from web designers in 3...2...1)
I understand the need for wineries to outsource their Web presence to third parties. It's just that the real measurement of a website -- its effectiveness in meeting the needs of visitors -- is (a) hard to measure, (b) not generally available for comparison, and (c) questioned by skeptical winery owners. So, important decisions about website design frequently come down to "Is it pretty?" or "Do I (the winery owner) like it?"