In half the cases parent presented, there were no "existing tools" to update(manual polyfills -> modernizr/6to5, script tags -> package managers like bower/npm, nothing -> eslint/type checkers, writing ad-hoc scripts for your own os -> grunt/gulp).
The rest of the examples are cases of merging two large ecosystems(web development and nodejs) together - that spawned browserify/webpack for bundling, npm for package management.
Many of these tools did try to update and adopt the new schemes. The grunt developers spent a lot of time optimizing their architecture in order to achieve the same speeds that gulp had. NPM has not stopped evolving - major version 3 tries to address a lot of the problems that yarn tackles. Babel evolves to adopt new javascript features.
I see the churn in development tools as a very natural progression, but that might be because I've been following it for a while and have seen and felt the pain points that each of these tools try to solve. I see a lot of effort within the javascript community to remain "compatible" wherever possible - for example, there are tons of guides how to migrate from grunt to gulp, there are matching tasks to ease your migration, etc. Library developers often release both on npm and bower, and craft elaborate "UMD loaders" for their libraries to allow for commonjs/AMD to exist side by side.
All in all, I find living with "javascript fatigue" to not be very exhausting. In our company, we have a wide variety of projects - some use literally no tooling, some use just bower, some use grunt, some use gulp, and our latest ones use the "modern stack" with webpack, etc. All of these projects are still actively maintained and their respective tooling stacks work and provide the benefit they were added for.
The problems the "newer" tools try to solve may not even be problems your development process has. If you've never really suffered performance problems working with grunt, then perhaps you've never felt an urge to migrate to gulp. Maybe you've never been bitten by npm's non-deterministic installations and poor download performance(I really haven't, so I haven't migrated to yarn, for example).
The magnitude of web development projects has ballooned in the last couple of years, and the tools are racing to handle it. It's not surprising that tools that are fit for a large web application with tons of code and a large team aren't really fit for a few javascript files to decorate a mostly static site built by a single developer. The churn is there, if you need it, but you don't have to use any of the newer libraries or tools.