Not without reason: Stardew Valley, which has been sold to millions of gamers, has been created using the free MonoGame engine. So ConcernedApe is giving back to the open source software which made his commercial success possible, like commercial parties should.
Gifts do not confer obligation. Copying deprives the original party of nothing. Absolutely nothing about free software requires or even implies any responsibility to “give back”. This idea that anyone making money with free software somehow owes the original authors anything (or “should” donate a portion of their profits) is ridiculous. If the authors wanted money for their software, they would have sold it instead o…
Someone built this and is letting you have it. For free. There is no legal obligation or law of the universe here, sure, but if you're in the top 1% of benefactors of this pro bono work, you have the opportunity to do some good and make sure that others, like you, get the chance to benefit from this free work in the future.
There is a pretty straightforward argument to be made that this falls under the "with great power comes great opportunity" umbrella of moral reasoning, since this work empowered CA to create the game that earned him a lot of money.