I'm not a professional web designer, but I've taken a few jobs doing it.
They do it because that's what their customers want.
I once designed a website for a dialup ISP. The default page template I made for them had one small graphic - their logo. Everything else was standard HTML and CSS 1. It was well organized, with the links easy to find for both customers and prospective customers, and it was classy, if minimalist. I included setup pages for four versions of Windows, MacOS classic, MacOS X, and even a page with info for Linux users (if you've never dealt with the different distributions in the days of dialup, count yourself lucky. It was a crapshoot.).
They used it for a couple months, and then paid someone else (probably a lot more) for a horrible, ugly clusterfuck that took a long time to load (I did mention this was dialup, right?), centered everything in a tiny column in the middle based on percentage (imagine that on an 800x600 screen, old but not uncommon at the time), had little information besides marketing bullshit, and required newer browsers than many of their customers had. They didn't even include email setup instructions.
These guys ran an ISP (albeit in redneck central). They should have understood the issues. Someone running a pottery shop? No chance.