I'm not very surprised, personally. There is almost zero incentive for these developers to finish their product. I imagine these numbers would be a hell of a lot higher if the system was set up in such a way that if, after a certain amount of time like 6 months to a year, your game was still unfinished then all of the purchasers would be immediately refunded.
But of course, that would require actual quality control standards at Valve to evaluate whether or not a game was fit to be deemed "finished".