The logic of the files being there are:
The package you install can require an installer program itself, nowdays there are many many installer programs, and versions of them. The Installer programs are not as backwards compatable as they should be, even when it is only a version change.
In fact you can install one package, and then install another that updates the installer, and the updated installer will no longer remove the first install :-) confused yet?
The Installer Program itself is required to re-run , when you wish to do an uninstall (or repair or re-customise) the installation, This entire program of the installer must exist, plus the package that is to be installed.
Luckily it gets worse, because it was not stupid enough before. There are packages which install installers, which install packages with installers. then the installer will run another installer like say the MSIs own installer. This of course does not include when a Installer is repackaged into an interleaved installer, to install some evil poop like a toobar.
Honestly it gets worse than this too, like installing an entire OS Subsystem because the programming subsystem is nessisary for the method of programming that they used to create the program.
so now you have the Installer, and all the install files, the installation package and all of its files, the System restore which is the instalation removal and all of its files, the driver backup and all of that repeated again, the system backup which is all of the files, plus many programs will also put all of the installation files in thier own folder.
The only thing we dont have is stability :-) we just have layers apon layers of mad attempts at self repairing.
Removing the "installers" (installation program) will mean that the installed program cannot be uninstalled, worse some wont install an upgrade, if the program cant be un-installed (using the original parts).
There exists also "Is Installed" triggers in the registry (various types), that has to be switched off to re-install (or lapped install). Plus other registry items which can effect all of the above.
Removing the "installation" (the installation package) will mean the program cannot be uninstalled, even if the installer program exists, because the installation package is required for the instalation. (plus registry items again)
Plus having all that does not guarantee that everything does get uninstalled, or that the program will even be able to repair itself with the installation package.
That is why I feel the need to run ANOTHER uninstaller :-) That tracks everything that changes, and can just toss it all out. Without the tracing the full installation (including first run), removing parts and pieces of it will mean it will not be easily removed. Without full tracing of an installation, removal of any of the multitude of parts and pieces, and layers of protection, and repetition, can not be done easily. Because full removal does not actually occur, this usually Leaves the user re-installing the whole friggen operating system from scratch to clean it up.
Sometimes you can Reinstall the original whole installation, (lapped install) to replace all the items you removed (+ all of the above mess), then the installation and un-installation itself will again operate. But Often this also requires resetting any items that can cause the installation to fail (usually in the registry).
the way to Safely remove the leftover junk, is to have a full image backup, and no intention of removing or repairing or updating those programs, other than the full image backup (retract).
To have fully traced the install, and used a full image backup. That way your traced install can be used to remove it, without all that stuff. (which is not without ramifications) Your FULL image backup can always retract your system back to where it was (before or after as needed).
I find that this method is way more effective than layer apon layer of moss growing on what will never be a rolling stone :-) but there is some requirement to know what your doing.
This post fully Illustrates why the Best method is not discussed or adopted to, there are just some thing, you should never have to see. :-)