This is 100% teams who set up their build tooling back in 2015 and haven't updated since. There's plenty widely used apps and libs that date this far back, and back then, IE8 compat was considered pretty important still, esp for products targeting enterprise/government customers.

Upgrading eg Webpack and Babel and polyfill stacks and all that across multiple major versions is a serious mess. Lots of breaking changes all around. Much better to just ship features. If it ain't broke, don't fix it!