Just speaking for myself: Given a limited amount of time, it is generally more interesting and fun for me to find new games and build/tweak them to get them running well, than to spend time dealing with installer and repository issues.

A second problem is that many of these games are just too large to be installed via application manager. A 30 MB game .deb will require >60MB free space on your root fs to install. There are workaround hacks for this, but making a robust one that would satisfy the 'casual user' who just wants to click and install is not trivial.