You care to elaborate? How do they seem ungrateful?
They put in massive amounts of time and energy into what they do, and like the rest of us they expect to be rewarded for their work. I don't know about you, but I don't go into work everyday to tell my boss "Thanks for letting me work here!" I go there everyday to do my job so I can get paid. You expect someone else to live by different standards because their job isn't conventional?
Honestly it seems to me that they are just sick of doing what they are doing. Their Minecraft videos aren't as entertaining as they once were, but if they stopped doing them altogether they would lose so many subscribers that they wouldn't be able to maintain the business they are trying to run, so it is a lose lose situation. This to me seems like a way for them to be able to get out of feeling forced to do something they are burnt out on. They can still make videos, but at a less demanding pace, and they won't lose as many subscribers as they would if they were to just quit doing Minecraft videos.
While yes, it would be awesome to play games and get paid for it, it would also be very frustrating to have to play the exact same thing everyday for years just to be able to continue to make videos for a living.