It's the demographic timebomb. They see the writing on the wall, that white people are not even at replacement levels while Hispanics and others are exploding in numbers, and concluding it's the safe long term strategy. And given the current state of affairs, they're exactly right. It is the safe strategy.
The same thing is happening in Europe. White people everywhere have been successfully guilt-tripped into willfully dispossessing themselves of their own civilization. Somehow, the narrative goes, this cultural suicide will make amends on some imaginary balance sheet.