It's not defeatism. It's identifying the problem, and identifying that political change should be the priority. When liberals are out there deregulating this shit, subsidizing the industries that contribute to the issue, and then saying it's the fault of the consumers, you can start to see why just telling people to cut down on their carbon footprint and leaving out that we should be advocating for environmental regulation, walkable cities, etc. might be an issue.

TL;DR Saying that corporations are the primary ones at fault isn't "defeatism", it's saying we need to do something about them. If you're such a doomer that you think corporations are invulnerable, that's on you.