Because it's easier on their mind.
Let's be honest here, if we wanted to do something about global warming, we'd have to change our way of life. And we'd have to change it big time. We (Europe and the US) use up more resources than the rest of the globe combined. Yes, including China. But they're trying to catch up. Should they ever reach us, it's game over anyway. That this isn't even sustainable in terms of resource usage, not to mention waste production, is, at least to me, obvious. If you disagree, do so, I don't give a shit.
So if you admit that global warming is a reality, you can do two things: Either feel guilty about continued overusing resources or reduce your consumption. Either is not really something people want to do. The first makes you feel bad (and we all know how troublesome this is to the fragile souls of our millennials) and the latter inconveniences you.
So it's easier to just wish it away and say it ain't happening.
Personally, I found a third way. I simply don't give a shit about it anymore. Yes, I'm convinced that global warming is real, the data I have available points to yes. But so be it. I won't live another 50 years, so I don't give a fuck.
If you want to save your planet, go ahead. Hell, I'll even move along. But don't expect me to waste any more of what's left of my time on trying to convince people that they have a duty to their kids. If you don't give a shit about your kids, how could you expect me to?