if quality would be so much better i would think people would buy even more. yes they could play the same game 20x but it sounds pretty odd to me that you buy less games if you have better games because few of them are enough then.
i think reasons could be
1. ps3 players are more likely also psp/vita players so they have/had to split their money/time for more devices
2. ps3 owners are more likely also movie fans with the blu-ray player so they maybe spend more time/money for movies
3. xbox players have more time. i once read ps3 owners are older on average which means more of them work and have already an own family and more xbox owners are students or something. ok older people have more money so it's hard to say which buy more^^
4. xbox is out longer which means people like me who own the 360 since launch buy xbox games for 7 years now. but i can only buy ps3 games for 5.5years (but i didn't own the ps3 from start)
5. xbox was not only out earlier, it also sold better compared to ps3 in the first years which means percentual, more ps3 owners have bought their console in the last years.
6. xbox is stronger as ps3 in usa, the market with the highest ratio which increases the worldwide 360 ratio more as ps3's worldwide ratio.