As a programmer that likes to paint, I think the difference is that web pages are a visually artistic medium. Artists that have unique fine detailed visions often feel the need to use low levels tools to perfectly achieve those visions. I guess this this applies more to individuals than teams however.
What is the value of reimplementing a bunch of this if it already exists? What's the value of having every developer learn some poorly-documented in-house system when we can use one that's already out there with full documentation, examples, references on StackOverflow, etc?