Then the software vendors are divided as to whether the software should be installed by "dragging" an image into Applications folder, or by running an installer of some sort, which is usually a package that installs other packages or some such. Then, there is a Apple store crap on top of all this, which, if I remember correctly, is also a package manager install.
Apple sucks at software. Mac OS X is stuck in 1995, as far as package management goes. The consumer does not care, hence Apple has no incentive to fix this mess.
Apple has a beautiful thing going: the "drag to install" and the self contained software images. Yet, they won't make any effort to regulate this.
P.S. The "uninstall" process on Mac OS X is usually some sort of a crappy shell script, which executes various "rm -rf" commands. The reliability and quality of those commands is left to the creativity of developers, that normally have no clue and write shell code by looking up examples on the Internet. It's a wonder uninstalls do not wipe out the whole disk more often.
What. A. Mess.