I'm guessing its because the designs are easy to make and many less experienced individuals tend to fall for these designs.
I typically find many scam sites to be similar to those "Get Rich Quick" scheme websites.
Typical website look:
-Huge fonts at the top
-Red text
-Highlighted text
-Underlined text
-Formatted like a huge letter
-A long list of testimonials in individual boxes toward to the bottom of the page, each with their own picture
-The end of the webpage is a signature by the person talking (ends the letter)
-A ridiculous list of PS, PPS, PPPS
-When you try to leave the page, there's some popup enticing you to stay with a special offer (and possibly a fake expiration date and number of packages left)
Of course, sites that look like these aren't always scams, though most are and I don't trust sites that look like this. I have seen a few sites that trustworthy sites recommend that look like this (though I never actually tried the products being offered myself).