Sadly it’s the constant barrage of breaking changes. It’s not just a version number bump and everything keeps running. Patch version breaks the app. Minor version breaks the app. Major version breaks the app (understandable in semver) but multiple inter-dependencies need to be updated in parallel.
So part of my job is maintaining a complex webpack build system, and I do get that it's not a small amount of work. However, the changes I see aren't wonton. They're typically aimed toward a world that's more interoperable and more standards-compliant.
For instance, Webpack 5 deprecated worker-loader in favor of a construction using new URL() and import.meta.url. Adopting this was a huge pain, but what we get in exchange is a system that can load workers using standard syntactic constructions that work under e.g. our test runner or in a NodeJS microservice. Something like that is worth the hassle to me.
As for minor and patch versions causing breaking changes, this has happened to me a few times. Terser Webpack Plugin 3.0.6 exposed flaws in a custom Webpack plugin we'd written for internal use, which I cannot fairly blame on anybody but ourselves. Babel 7.5 broke CommonJS targets in a way that wasn't fixed until 7.6, which was truly unpleasant and something I hope to never deal with again. TypeScript doesn't use Semver and usually has breaking changes in every point release. For the most part, however, semver has proven trustworthy.
The Darwinian foment that characterizes the front-end ecosystem has created the best tools I've ever used. When I look at the quality of tooling available on stacks that prioritize stability, I am not jealous.
I maintain oodles of npm packages, several of which enjoy over 5 million weekly downloads. They have dependencies, and I cannot support your claims.
Sure - some packages in the ecosystem will not follow semver, some will let breaking changes sneak in where they aren't supposed to be. But you'll find this in any pluggable or package manager ecosystem. Please, measure your frustration and choose not to paint with such a wide brush.