Why dont buyers leave feedback??

I've not had many sales, but I'd say half or so I struggle with getting the buyers to leave feedback. I know eBay "reminds" them to leave feedback, but obviously some of them ignore this, or simply can't be bothered.

 

When I buy an item, I leave feedback as soon as I know if the item is any good or not. Are any other sellers having problems with this? Its got so bad with mine I'm having to contact buyers to remind them, and even then, they're still not leaving feedback!

 

Everyone who does leave feedback for me only has positive things to say - and I have a 100% score - so why do some buyers simply not bother??