The developers themselves are often not the package maintainers. Before a package is published or updated in one of the official Arch repos, it has to be built, tested, and sometimes patched (which is why you see a -1, -2, etc. appended to the package version), in order to work correctly not just on its own but in an Arch system with Arch packages that it is likely to encounter. The process is not as thorough as Debian for example, but it’s still the responsibility of the package maintainer. If the package is still in early development, deprecated (e.g. wine32), an out-of-tree kernel module (e.g. xpadneo-dkms), or is meant to be built from the latest available commit (any number of *-git packages), the AUR is a convenient way to share PKGBUILD files rather than have the user build the software manually based on a readme, if it even includes build instructions. The PKGBUILD is then ingested by makepkg, which both configures the environment and builds the software, and outputs a package that can then be installed and managed by Pacman.

The caveat is that packages built from the AUR are not vetted by any package maintainers. They can have bugs, they might depend on outdated or no-longer-existent packages, or might contain malware.

If you want to be really pissed, read up on doctor disgraced former doctor, lifelong charlatan and grifter Andrew Wakefield. https://en.wikipedia.org/wiki/Andrew_Wakefield

He abused autistic kids in a medical experiment that was meant to convince both the public and the scientific community that a specific combined vaccine (measles-mumps-rubella, a.k.a MMR) could trigger some kind of bowel disease that causes autism, all so he could peddle his own alternative that is three separate shots.