I taught myself html using Blogger back in the day. Now, most sites really push templates and don’t allow for much customization. So, why html and CSS? It helps introduce basic coding concepts. A lot of what html and CSS cover is expanded upon in JS. The importance of it also depends on why you’re taking the lessons here.
If you go into web design, it helps you learn to code from scratch. A client will hire you because they want a professional looking website. What Wix and many hosting sites offer is not professional or personal - it’s not catered to an individual company. That’s why they hire developers.
If you’re looking to get a job with a company, it won’t go very well if during a technical interview you ask for a template to help you code. They want you to show that you understand and can implement the concepts you’ve been taught without help. Finally, learning code opens up more possibilities for design than what a site like Wix can offer.