The answer is usually that whoever set up webpack grabbed the configuration from another project, and nobody wanted to touch it since then because it worked and it looks scary.

Young developers don’t know what they don’t know. If they don’t see editing the webpack configuration as part of their job, they’ll probably just never touch it.

And that means it’s your job to go fix it! Compiling down to es5 can have terrible performance implications for the resulting application - particularly if any of the bundled code touches typed arrays. There’s some free performance wins sitting on the table. If your coworkers are too junior, it falls to you to fix the build.