Because of pragmatism. Ask someone to make a significant sacrifice and they'll tell you to piss off. Hell, I'll use myself as an example: try telling my SO and I that we can no longer fly a few times a year to the alps for winter sports and I'll just be rude and laugh at you. Sure, people were happy before they had good meat available, they were also happy before polio was eradicated but now we're happier and nobody's going to sacrifice that.
It's suggestions like these "oh, people should give up a lot of things that they enjoy" that makes me look pretty unfavorably towards modern environmentalism: it's not going to happen, we need pragmatic solutions that will actually work in the real world. This is where environmental engineering comes in but God forbid someone suggest us actually making intended changes to our climate to fix problems to the current breed of environmentalists without them foaming at the mouth. Thankfully I rest easy in knowing that eventually this will be an inevitability, as soon as it becomes obvious that the idealistic path is a dead-end.