Effort is beyond abilities of average ISV

And if they use a standalone installer, they have yet another application 
they have to look after to stay secure.

If they put up with the burden to add the repository, verify the 
repository signing key and install it with the distributions package 
management system, the updates will come in like any other security patch.

ISV packaging will lead to even more code duplication with libraries like 
libpng etc.

IIRC openSUSEs One Click Install provides a way to the casual user to add 
repositories and install software with one click.