that's easy to explain. the majority of "pirates" are schoolkids that don't have the means to pay for a game online. they buy games in stores from time to time but the realization that they can get any game for free, for 12-16 year-old kids...
anyway, the point about pirating demos is also correct. it's habitual. they tend to check their favorite torrent trackers for all gaming news. so as soon as something pops up, they only need to press the DL button
This is a biggie, more importantly the part of them having no means to pay for a game online. I suppose the sellers could start promoting kids to start stealing their parents Credit Cards and using that, but I think that would create more problems then are already in existence.
Honestly I'm quite surprised that this is still a problem.
I know a lot of local services that can be used without any real age-checks by cell phone transactions.
Then again maybe expecting the youth to sacrifice so many SMSes is unreasonable.