Because everything is complicated now. To code your own website you have to learn hundreds of hours of web technologies. There is no clear way and a lot of old, now wrong, advice in old blog posts. This limits the creativity that can be expressed a lot and is probably the main reason why "everyone" is now using "platforms".
I disagree. its no harder to program today than it was 15 years ago. in fact it is easier. its less meaningful....less satisfying....and has been eroded by copy-cats and everyone spinning up a product instead of a passion project.
I created my static site with scripting, CSS, and HTML skills from the 1990s, an oughties Comp Sci degree, and w3cschools. HTML5 makes it scary easy to make a pretty site which works on a wide range of screens (no more full-width columns of black text on a white background!)
Because everything is complicated now. To code your own website you have to learn hundreds of hours of web technologies. There is no clear way and a lot of old, now wrong, advice in old blog posts. This limits the creativity that can be expressed a lot and is probably the main reason why "everyone" is now using "platforms".