Why Do These Sites All Look the Same?

I've been wondering this for awhile: why do the websites of so many eBook and DVD programs look so similar? Most...

• ... are really LONG pages, with lots of scrolling to reach the bottom.
• ... are one centered column, with wide margins.
• ... use the Arial font, with big headers.
• ... highlight key phrases in yellow.
• ... are signed by the program's creator, often including a scan of his signature.
• ... are peppered with testimonials.

Here are the examples:

• Complete Speed Training, Latif Thomas and Patrick Beith
• Burn the Fat Feed the Muscle, Tom Venuto
• The Vertical Jump Development Bible, Kelly Baggett
• Muscle Gaining Secrets, Jason Ferruggia
• Precision Nutrition, John M. Berardi
• Ultimate Insider Speed Training Secrets, Tim Kauppinen
• Matt Furey

It's odd. I wonder if they all use the same storefront software, and whatever templates it provides, or if they all read the same book on selling stuff on the Internet. Or perhaps they read the very interesting article, The Surprising Truth About Ugly Websites. Not that all these sites are ugly, but all have at least one or two elements that fly in the face of traditional web design, and a few have a bunch.

I'm not making any value judgments about the programs themselves, as I haven't bought any of them, but I will say I've heard great things about several of them. I've read several articles online by Thomas, Beith, and Baggett, and they all make good sense based on what I know (Google those guys - good stuff). Venuto is well-respected, Berardi is revered (I've heard nothing but great things about his program), and I really enjoy Ferruggia's weblog. I have no point, I'm just curious how they all came to use the same marketing style.

P.S. I don't know anything about Kauppinen. I've read and enjoyed a few articles by Furey, but his product list and prices give me significant pause.