Not an evolutionary biologist, but hear me out.

Let's say kindness is, say, paying attention to someone else and voluntarily spending resources on them, ideally at some expense to myself. Right?

Okay. Then let's say I also understand that I depend on you and you depend on me. And, if I act tyrannically and make you upset enough, you'll retaliate. And since there's only one of me and many of you, that strategy is not survivable.

Cooperation is, though. And better than cooperation is admiration. Better to be voted in to lead than to take the mantle by force.

So kindness is perhaps one way to show others that they are safe with you, that you have their best interests in mind, that you're willing to sacrifice for them, and that you're a good leader, because of your capacity to attend to (many) others. It's why we have leadership books titled things like, Leaders Eat Last. I haven't read the book but I innately know what the message is. That's no accident, that's hard coded somewhere in my mind and body, and I bet you have the same sense.

Why wouldn't that, given how much more survivable it seems, be transmitted genetically?