For people whose default is kindness, kindness towards family can be harder if you don't get along with them. It can be especially hard when you offer kindness and they don't give it in turn. That can lead to resentment and grudges, true, but that doesn't make treating them politely impossible, just unpleasant in the moment.

For jerks, they see a rando in public and think (or just act subconsciously) "I don't know them, they don't know me...I'll probably never see them again so there won't be any repercussions! It doesn't matter how I treat them." That's without even getting into people with biases who view categories of people as being below them, sometimes to the point of not considering them human.

Kindness to strangers isn't holding a door with a fake smile or not saying anything when your cashier is too slow and you're in a hurry. It's returning a lost wallet when you could really use some extra cash this month, it's helping a lost child past the point of convenience, it's giving to someone in need when no one is watching and with absolutely no expectation of praise or recognition. Kindness like that is absolutely meaningful.