Simple, job security. It would put lot of FrontEnd Coders out of work - not all, but many of them. So it is in everyone's best interest to keep coders coding, designers designing and have as little overlap as possible. Silos keeps the wheels turning.
So there has been a collective badmouthing about how WYSIWYG tools create a codular mess, and then they proceed to create similar codular mess by hand (pull this library, glue, pull that library, glue..etc). After Bootstrap, webdesign is dead. Every single website look the same. If not literally, then same blocks re-arranged (Hero Image, Sticky Nav, Round Profile Pictures, Contact Map, Footer).
I remember Dreamweaver in late 90s, it democratized webdesign, as a design student, I was able to pretty much draw and create as simple or complex I want. In 1998 Dreamweaver promoted "Roundtrip HTML" to quell the backlash - but dev sentiment killed a lot of momentum it had (They still badmouth it). Semantic Markup, Tables... unmaintainable, and then proceed to have the same convoluted polluted code. Was it perfect? NO... but it was not FrontPage or Word>HTML. It is 2017, and with the technologies available today, there should be multiple tools to cater to UX tooling.
For most designers, learning to code kills the spark of creativity, and puts you in a templatized mindset (Hero Image, Logo on top left etc).