How come some buyers don't leave feedback?!

I can't understand why a buyer would not leave a feedback.. at all... Even if negative, or at least say something!
As a new seller, it's hard to pump up the live portifolio when people just don't leave any feedback - Fiverr just marks it as completed, as their feedback time was over...
Also it's hard to know whether they liked it or not, and that's the most important part of the feedback, at least for me.

So please, buyers, don't forget to leave a feedback before due!