Because, while as low as 12 FPS is enough to create the illusion of movement, it's not enough for it not to seem like jerky movement. The 24-30FPS is a sweet spot in film, but that's because they have motion blurring, which makes up for the deficiency. But said blurring was at first prohibitively expensive (in computer resource terms) in PC gaming, and now, people have gotten used to the crispness of 60 FPS. Why go back to 30 FPS and have to add a filter, to get a blurrier (or, worse, jerkier) result?
And, yes, as games are designed to run at 60 FPS mostly (some parts of console games being an exception), the game itself often does slow down if you are below that, as it means your computer itself is slow.