...why not?
There is also a bit of an art to such short videos. It is just another way of bringing out some creativity to get the point across with so little. It all depends on whose Vines you are watching (could be someone actually into it, and who does cool stuff... or could be someone who should really be spending their time on Snapchat instead). Vine also seems to be home of comedic timing
It's not really something to be understood as an audience, mostly. If you're into it, you're into it. If you're not, you're not. Depends on what you like to see!
Oh, and not to mention how we all love to waste our time on the internet. Sometimes 6 seconds at a time xD