Also... If you are a big company, and your reputation could be ruined if something goes wrong (remember HF20?), would you rely on something that is decentralized? On something that can be changed by some people who are not under their control?
I mean... 100 witnesses x 50$/month worth servers = let's be generous, 10.000 $/month. 10.000 $/month is a peanut for even a decent company, not giants.
What do you think, if Instagram needs a coin, would they build their own coin, make HF overthrown impossible and keep all the servers under their control, or they will just believe that people are good creatures and that they would never, ever do something evil against them?