It is that easy because in Mac OS X, applications files aren't simple files at all, they are completely self contained archives. Unlike your average Windows or Linux program, that needs to scatter support files all over your desk, Mac OS X has each of the programs keep all of their support files right with them, in their archive. Hence, the app file is pretty much wholly self contained. To prove this to yourself, go your Applications folder, select any application at random, right click it, and select "Show Package Contents". Very illuminating!
As surfwax95 points out, this applies to most programs, but not all. Some of the real "biggies" like MS Office, Adobe Photoshop and the like, DO scatter files around somewhat. But by and large, most programs don't and that is why you generally don't have installers. You get the program in a .dmg file, and you can literally run it right from there if you wish - no installation needed.
Installation on a Mac is generally just a matter of dragging the file to your Applications folder. That is it... and there is no magic there either. All that does is make it available in one fixed spot (so things are organized) and makes it available to all userids on the machine, not just your userid.
The only exception to the "no support files" rule is that most apps keep preference files (the modern day equivalent of those once ubiquitous .ini files that Windows programs used to keep) to record the permanent settings you have selected via their preferences dialogs. You can manually delete them yourself, or use an application like AppZapper to do the whole thing (trash the application file and all of its preference files) in one stip.