“Liberals” (and I’m not even getting into what people here define as “liberals”) absolutely can confront capitalism in order to address environmental concerns…

I don’t think you’re grasping the statement. It’s not a philosophical limitation, it’s a systemic one. Liberals are dependent on corporate donations to maintain the level of messaging necessary to remain competitive with the hyper-capitalist right. It is not, in fact, merely the case that they don’t want to, it is the case that even if they wanted to they would lose the ability to speak in a system where the only speech that matters is dollars and cents.