I think something people haven't mentioned yet is that games are so much a digital media now that where I used to be able to keep infinity games at all times in a CD book, I now have to selectively decide which games get to occupy my limited hard drive space, and installing a new one means uninstalling another, and waiting to redownload it, and between my limited drive space and less than amazing network speed, those can absolutely influence what I'm able and willing to play at any given time.
It's more logical to keep ten games I know I like installed rather than choose one of those to cut off in place of a new unknown quantity.
Also, compared to other major sentiment I see in this thread, I actually quite like tutorial sections of games. I'm often very interested to see what the game itself has in store in terms of exactly what mechanics and systems it contains and how they execute them, and how that stacks up compared to reviews or word of mouth, which are often vague, biased, or missing portions of the experience.
After I fully understand what a game is trying to do, I fall off the wagon often times as it sinks into a routine instead of a novel learning experience, or maybe I actually love it, but standards continue to increase as more and more novel ideas and fusions of genres are created and become existing products. It becomes more difficult to make something that's not something you've already done, but slightly worse or only slightly better.
I still "get into games" plenty, but it doesn't happen quite as often, and it's the "sticking with" them that becomes more desired and elusive.