I mean, this indie developer sold more than lots of (most?) AAA studios, and AAA studios have a lot more operating costs than an indie. Donating one dev year is a lot easier when you sell 50 million copies and hire no one than when you sell a couple million copies and have 100 employees plus investors. (I have 3000 hours on this game, so definitely not biased against indies or something.)
These studios have profit margins in the multi-millions, they could afford a symbolic $100k donation. Instead they choose to push their developers through another crunch cycle.
They typically pay a lot more than $100k for Unreal and the likes or employees working on their in house engines.
Oh and these studios often lose money instead of having profit margins in the multi-million, see Ubisoft.
But ConcernedApe uses Monogame for Stardew Valley, the studios don't use Monogame. Why would they donate?