How many times do we have to learn the same lessons over and over? Small tools that compose together is infinitely more flexible than any plugin based tool can possible be.
That is, the point of using webpack isn't that it's the best way to optimize a bunch of PNGs. The point of webpack is that it can parse your dependency tree and figure out which PNGs are depended on by a given entry point. If you also want your PNGs optimized, you can do that with a webpack plugin or a separate tool, as you like - that's orthogonal to the matter of why it's useful to import PNGs into JS.
As such the point of requiring a PNG into JS (or CSS, or a Vue/React component, etc) is to tell webpack there's a dependency on it. Then that image will get pulled in to the build process for entry points that need it, and not otherwise.