I think part of it is that the web developer community exploded. More developers = more people trying to make their mark = more churn. Everyone wants to be the person who "fixed React" or "reimagined routing."
But when you're actually building a product that users depend on, you realize how much of a tax this is. Every framework "upgrade" that breaks things is time NOT spent on features, user feedback, or actual problems.
The irony is that the best products are often built with "boring" tech that just works. Instagram ran on Django for years at massive scale. Basecamp is still Rails. These teams focused on users, not on having the hottest stack.
What frameworks/tools have you found that stayed stable and just worked over the years?