Because I've wasted my entire weekend clicking cookies.
That's right. There's a game in which all you do (many of you probably already know of it, and I apologize in advance for this) is click cookies. Clicking cookies gives you more cookies, which allows you to buy things that will auto-click cookies for you, which allows you to buy upgrades to generate cookies faster, which only gets put back into buying more things to click cookies for you.
I have literally spent my entire weekend (I was sick most of it, so I couldn't really move around a lot anyways) on this game. I literally have every upgrade and achievement. I have over 100 of every cookie-generating thing available. My CPS (cookies per second) is... well, a lot. I have over 200 heavenly chips (prestige levels) giving me a bonus 400-something percent to my cookie production multiplier.
So, you get the idea. This stupid little game has sucked away entirely too much time, all because it knows how to make you feel like you're accomplishing something (when in reality you're just clicking a virtual cookie over and over).
I dare you to try it. Cause you know you want to. The cookies' call is too strong to resist.