In the olden times (e.g. 1999) you would write HTML by hand (notepad.exe) the same as people write markdown today. It then works as a single thing to move around, I don't need to bring a second thing with me so reorganizing simple HTML files is easy. So if I am just going to write a one off document HTML with images and tables was the easiest (see very many professor websites, geocities, angelfire, etc). Once org-mode became the better version of this because it could export plain HTML I shifted to that personally but if org-mode didn't exist I probably would still be writing HTML directly when it comes to just typesetting tabular data vs dealing with Word/Google docs tables.

Automating writing HTML is also easy as you can just write to stdout directly, and if you are comfortable writing HTML by hand directly this is a simple uplift to automate things you did by hand.

This is why jQuery was so popular because you could just add it in to this vanilla HTML in a nice way. You didn't have to stand up some JS scaffolding and toolchain to get there. Even jQuery can get you to a situation away from WYSIWYG because you don't just need to render the DOM you now have to have a JS runtime evaluation. In some contexts you have something that can write a DOM from HTML but you have no JS, it was only when chromium embedding became popular and normalized you got both the HTML and the JS together, for a long time you only got the HTML.

Finally, direct HTML will work as-is forever but JavaScript is a moving target. Some terrible websites I wrote in raw HTML work today after 20 yrs (admittedly 1024x768 optimizations didn't age well) but many JS things I have written have not had that longevity or require standing up a long dead development framework to get going again.