Suppose there were no taxes (which only makes things more complicated) and suppose I ask ten people if they want to get money by cleaning up houses for 8 euros per hour. They agree. Now I on my turn ask the people where they work 15 euros for cleaning their houses. This is basically the same as saying (after some initial searching for clients) that I get money while doing nothing. So isn't making a profit an unfair (in the sense that I, the mediator, has to do relatively less work, and make more money; I was always taught at school every child should get the same piece of the cake) way to make money? After all the people whom I asked, théy do all the hard work, while I do (compared to them) very little. The same can be said for very big corporates.
How come they accepted the 8 euro offer if they could have gone to the people who pay 15 euros as well? Could it be that matching the suppliers and the customers has same value? If what you are doing is truly trivial you will get nothing. If what you are doing is not trivial but relatively easy then you will soon have a competitior who is offering 10 euros to steal your workforce. And competition should eventually price the value of your activities.
Of course the real world is more complicated, but in your example the work of the matchmaker has value.
In response to the edit:
"the mediator, has to do relatively less work, and make more money"
If this is the case the cleaners obviously do not need the mediator and will not accept his sub 15 euro offer.