The people who reinvent it think "Wow! How can this not exist? It's such a good idea and we're first to market!", failing to realise that they didn't find another competitor because so many other people have built the EXACT SAME THING and failed and shut it down.
Here's the last time I said the exact same thing: https://news.ycombinator.com/item?id=7631094
about this: https://news.ycombinator.com/item?id=7628532
My personal version of this stupid concept was http://www.jobbountyhunter.com (hey the domain is available! snap it up!) and here was our version two - even better! http://web.archive.org/web/20090317010055/http://jobbountyhu... which I failed at about six years ago. We even went back for a second bite, rebuilt it and wasted more money thinking it would work if third party website published the job ads and collected some of the bounty. Strike two!
In my hometown, in my personal network of connections alone I know of two other people who built the EXACT SAME THING too, and also failed. I am certain there have been many other attempts and others will continue to build this thinking it's a good idea.
It's a stinky idea at its core because people don't like selling their friends and acquaintances. If you are the entrepreneur who built this then save your time and money, shut it down now and build something else whilst you have the time and energy and money.