Fallback support is a legitimate reason for additional code being in the bundle, but it's not 'bloat' because it's necessary. In an ideal world every website would generate ES5, ES6, and ES2025 bundles and serve the smallest one that's necessary for the app to run based on the browser capabilities, but that is genuinely quite hard to get right and the cost of getting it wrong is a broken app so it's understandable why devs don't.

The other two, atomic architecture and ponyfills, are simply developer inexperience (or laziness). If you're not looking at the source of a package and considering if you actually need it then you're not working well enough. And if you've added code in the past that the metrics about what browsers your visitors are using show isn't needed any more, then you're not actively maintaining and removing things when you can. That's not putting the user first, so you suck.

Bloat is mostly added by package authors, not website authors. And they can't know who's running it and can't look at the metrics. I doubt many website authors directly use isEven or polyfills.