You're absolutely right, that labor creates capital, through the addition of value. For instance, a painting by Picasso might have cost $200 for the canvas and paints. That same Picasso sells for hundreds of thousands of dollars, due to the value added by the artist, compounded by the value added due to scarcity. All of these things add up.
An artisanal loaf of bread might contain $1 worth of ingredients, but sell for 5x that cost, again due to the value added by the skills of the baker, combined with scarcity (you might not be able to buy a loaf of bread that great at Walmart.)
Marxist theory purports to claim that work is the driving factor of economics, while capital is static, but nothing could be farther from the truth. Value i.e. capital, is CREATED by work. Capital is created. Money is "made." There isn't a limited amount of capital. Capital isn't a big pie that everyone has to share, and if one person gets a slice, that leaves less for everyone else. Capital is created by increasing value through work, effectively creating more and more pies.
More than anything else, this shows that the leftist hatred for success that began back before the 1960s here in the USA is still gaining momentum. A perfect example of that line of thought was expressed in the song by "Ten Years After," "I'd Love to Change the World." "Tax the rich, feed the poor, till there are no rich no more." In other words, It's better to destroy the rich than to raise up the poor.
Mark
The communists would have us believe that the working man, or "labor" is the man who lifts heavy objects or digs the ditch with the strength of his hands.
They do not believe that a man like Donald Trump, for example, through is machinations, while not of the physical type, creates things. When the Donald is all said and done, there stands a towering skyscraper where nothing existed before it. This in turn enables the effort of others and provides for them, both during and after it's creation.
This, I think, is a small illustration of the non-static nature of labor/capital which you describe and the Marxists refuse to acknowledge.