Because the barrier to entry in programming is zero. Or near enough to it. Code on your free time, learn php on your free time. Suddenly you are WordPress developer.

Learning a trade can require very expensive tools, often time as an apprentice or journeyman, and learning at the very first stage of your career that your labor has value and you need to charge for that.

I can hire a programmer from anywhere in the world and often incredibly cheaply. I can't do that with a tradesman, they actually have to be local, often have more work than they can ever get done, and know that I can't outsource the construction project to someone 1,000 miles away.