Because HTML and it's various implementations/versions are a Byzantine mess.
'Web development' is not so much development ... it's about knowing a myriad of little, innocuous details regarding various platform, api quirks, toolchain bits etc..
The 'development' part of web-dev is usually quite straight forward - you're not doing complex AI etc..
And fyi - development is 90% struggling through things you don't understand, bugs, hiccups etc.. A good chunk of the code you write like English. It's all about working through the pain points, making 'aha' realizations about how you structured something, or about some crazy API quirk. That's the job.