Because it's easy to make good-looking things in Flash. Yes, I know that Flash is bad. But you asked the question and that's the answer.

it's a wix site - simple, online sites using flash. The guy is smart - he's using a simple tool that gets the job done, not spending his time writing a custom CMS.

That's a skill that I need to get better at: pick the tool that gets the job done _and_ takes the least amount of time. Then go do other things.