Just remember the days on whale hashflare and genesis did make such offer on where you do able to have a lifetime contract and now they are limiting already.Why? They cant sustain their business if they would decide to continue to offer such payouts or returns to their investors.
That's partly the reason. Another reason is that most of these schemes in their initial days offered these 'lucrative' contracts as a way to obtain capital to purchase actual mining equipment.
It basically means that they didn't mine anything back in the days, but do so right now. Till what extent is impossible to find out without them releasing any information, but definitely not on a 1:1 ratio.
Till this day GM has refused to publicly share their main addresses they receive their mint subsidies on. If you don't consider that a shady practice, I don't know what is needed to convince people.