Whatever good there may once have been in leftism has long since been used to smuggle in greater evil, and the chief evil is materialism and the separation from God with the subsequent celebration of man as his own god. Thus, the concept of the good is changed radically. No longer does it relate to spiritual understanding and focus and to one's relationship with God, but to the human being as it exists in this world. And no longer is the good a matter of objective reality, something with its own independent quality and truth beyond the merely human, but what bring most benefit to most people in a purely worldly sense.
This is why when you talk to a committed leftist, you talk at cross purposes. His idea of good revolves around what increases happiness and reduces suffering in the here and now. He has an enormously limited view of what a human being actually is and that deforms his whole judgement. Even if he is religious or spiritual in some way this will always be subject to his determining leftism, and leftism is fundamentally materialistic in that it sees man as he is in his fallen, almost biological, state and not as a spiritual being.
The question then arises as to why is the leftist like that. Is he just labouring under a delusion through no fault of his own or is his attitude actually a reflection of his own mind and will? For some it may be the former but for many it is the latter case. A leftist philosophy (ideology is a better word) is the outcome of the wilful rejection of God.
That much is clear but I believe there is often something more and that is a lack of imagination. It is amusing to think of the howls of outrage one would hear from leftists if they were accused of lacking imagination but what other conclusion can you reasonably draw? Imagination is not just conjuring up and playing with a bundle of images and ideas. It is sensitivity to the higher worlds. It is openness to a greater reality. It is, in its proper sense, a spiritual faculty and if you have it you will see that spiritual reality exists. You will, at the very least, be strongly pushed in that direction even if you hesitate to embrace the notion fully because of the limited presentation of religion in this world.
This takes us to the great problem with many church-going Christians which is that they too lack imagination so their idea of religion is restricted to the moral world or to the world of belief as stipulated by authority. But this is a desiccated kind of faith that does not inspire or transform. It leaves its members in the world. Imagination is the only thing that can get us out of this world. It is not enough on its own. It must be coupled with a proper religious understanding but it points to and begins to reveal the reality of higher dimensions of being, and a refusal to acknowledge these higher dimensions certainly indicates an unimaginative mind which is ipso facto an unspiritual mind.