The short answer is “because the good ones are worth it”. Cheap solutions are seldom the best solutions. But that even skirts a few issues.
Recently, I quoted a client my price for designing their website. I considered it a bargain (considering it was a fairly complex site), but their resident web guy felt it was too steep, so I encouraged them to shop around for a better deal. If they found one comparable to the work I would be doing, I was willing to match their price. He came back to me with a company that was willing to do it for $150. (Mind you, the client wanted two websites; one for each of their radio stations, one of which focused on local news and sports, with online streaming, personnel profiles, RSS feeds, blogs, etc.)
I checked out that company’s work and found (without surprise) that their work was sloppy, unprofessional, and completely inadequate for the business. I wrote back to my client with an assessment of the company’s work and pointed out several red flags. At the end, I likened a $150 website to a $20 boob job.
A good web designer has taken years to hone their craft. On top of learning the latest versions of several coding languages, CMS platforms (Content Management System), and trends, they also need to know how to use all of that to provide good SEO, a helpful and pleasant user experience, a solid strategy on how to use the site, and make it all load and work as quickly and efficiently as possible on any device and any screen size.
Also, you need to consider how valuable that website is going to be to you. If by having the website, you’re seeing an increase in business, it’s likely that the website will have paid for itself within a matter of months. For example, one of my clients had a website that was designed by a friend for next to nothing, but it looked shoddy and unprofessional. I came in and simply overhauled the look and organized the information a little better. After the launch, they began getting business based solely on the fact that their website looked really professional compared to their competition. Within two months, the website had paid for itself more than ten times over.
All that being said, can you still get a good website without paying a lot? Absolutely. Some endeavors don’t need the kind of site that I’m describing. If you can represent your band, restaurant, portfolio, etc. effectively online for $300 or even for free, by all means do it. In the right hands, there are several templates, platforms, and site builders that are completely adequate for many businesses. But if you find that your needs exceed your abilities, take a second and consider what that missing knowledge and expertise is worth to you.