the OP probably would be coming in from a W!ND0$ environment, pardon me if i'm wrong.
out there is just download the software and double click and follow instructions and the software is installed. So that is the reason most Distros brought out their respective software package centres, one click and enter password and voila software is ready up and running!
How many people even use the apt get command now a days??? like to test, very rare! So everyone wants the one click install and use mechanism. Unless you really want to get your hands dirty people will still take the safe route.
Also linux runs on most old hardwares. and over its more safe and secure.. who want to run a resource hungry anti-viru$ software on a old piece of hardware.
These are my personal opinions only. others would differ though!