Website dev and web app dev.
I don't think they are mutually exclusive, but website dev is about styling and navigating into documents. It's stateless and the variety of content is unlimited. I don't think there is a "problem" here, it works, has good tools for what it has to do.
On the other hand, there is web app dev, and app dev is messy at present. My guess is that is lacks governance, it's not necessarily a bad thing, but it means things are taking time and must prove themselves to gain traction and become standards.
But yeah, doing something like a calendar input, with really perfect behavior, where you can navigate the calendar with the keyboard if focused, that is accessible, works on all device… is insanely hard and not solved by the platform yet, so everybody try something different.
Also, a lot of people that are more authors than programmers are "hacking" "app like" functionalities into their website, and it can get messy very quickly. No pun intended.