Also remember lot (most) of Marxism predates Darwin and evolutionary biology. Even though Darwin and Marx were contemporaries. Since left is still quite based on Marxists ideas, it just does not account for many evolutionary biology theories and hence tends to find reasons which do not explain things correctly.
I would take it one step further. Marxists/Leftists generally discount time as a variable entirely. It's all about addressing a concerns in the present. There's very little to no considering to the impact of time. There are two ways this disregard of time manifests itself. First, is what you mentioned, in that there is no consideration for the value of wealth building, which (1) takes time and effort, and (2) produces the greatest returns over multiple generations (Basically, every one of us are standing on the shoulders of our ancestors). The second is preference for distributing the fruits of labor in the present equitably instead of using the fruits of labor to create greater returns in the future.
While not specifically about time, the difference principle in Rawl's Original Position is also relevant. Under that principle, inequality is permissible if it can lead to the worst off being better off than they would if there were equality. This principles presupposes that time is considered when trying to maximize well being for the worst off. If you want to maximize the well being of the worst off in the present, you implement equality right now, but that may be worse for the worst off in the future.
