¿Por qué el desarrollo web es complejo?

Porque se basa en una montaña de errores. Eso podemos leer en Why is web development so complex?:

Mainly because it’s built on a tower of mistakes.

HTML was a screwup, CSS was probably worse, and don’t get started on JavaScript.

If the web was to be redesigned from scratch, with no thought to backwards compatibility, it would be nothing like what we have today.

It’s complex due to simply piling on more cruft onto something that should have been thrown away and restarted decades ago, but we’re stuck with it now.

Start simple, just do a small amount of HTML, and style it up with CSS, and work your way up from there.