I mean, if you think about it, DLing demos is kind of a pain, and it requires that you connect to the internet and scrolling through some selections.

Why don't console manufacturers just pre-install demos in the hardware like they do with some games? It would bring more exposure to current titles in the library and possibly give the older games a better tail end in sales if new console owners become more aware of their existence.

Is it because of the belief that demos cause lower sales? Or is it something else?

People would whine about unnecessary demos taking all the HDD.

Plus the consoles are manufactured long before you get your hands on it, the games would already be old fashioned.

That doesn't mean that you can't put in an old game. People might mess around with it and buy a copy.

Some old games that went noticed before could get a lot more sales if a demo was included in the system.

Because it would be necessary to put one of these age stickers onto the box.
Killzone demo preinstalled? Suggesting the whole console is for mature people only.