1 - The quality of images are not that good. Some are okay, but overall not that good.
2 - Many microstock sites have large volume of decent looking images and you can always find something decent.
3 - You end-up spending more time on these sites trying to find a decent image than you save money.
4 - Photography (quality) is arts and craft. Because of the higher supply the price have come down but they still need to make money.
5 - Model release? Just looking at one of the sites [1] one of the images had a photo of a skater [2]. I'm not sure if the skater had given a model release or not. But at least with more reputable stock sites you know there is always a model release required.
[2] https://s3.amazonaws.com/ooomf-com-files/7erBZvZMQ2mmuFQ10vc...
edit: formatting