There's a long history of distributing source code already partially processed, and for decades, this has been what people expected when they download a source tarball. Newer generations were introduced to open source through github and are less familiar with this, but they also are rarely interested in source tarballs — they're interested in either the source code to contribute to (in which case they'll git clone) or in a binary distribution.

Really the main purpose of source distributions is for packagers. Especially when you're creating pkgbuild scripts like for archlinux's AUR or gentoo or slackbuilds or macports or BSD, all the tools needed to build the source code become build-time dependencies for your package, which is annoying for people installing it.