Maybe we should try looking at this problem from the demand side.
Suppose that I bake pies and sell them. My pies come in two different varieties, apple and brussels sprout. It takes exactly the same amount of labor to bake an apple pie as it does to bake a brussels sprout pie. (The labor that goes into producing the apples for one apple pie is exactly equal to the labor that goes into producing the brussels sprouts for one brussels sprout pie, and my work to produce them is identical).
According to Marx, my pies should cost the same amount. But in the real world, my apple pies command a premium price (they’re quite tasty), while I can’t even give away my brussels sprout pies (even when I tried that “buy one apple pie, get two brussels sprout pies free” deal).
Here the scarcity of the product is not an issue. It doesn’t matter whether there are millions of brussels sprouts available or a limited number, if no one wants brussels sprout pie, no one is going to buy it.
You’ve grasped an essential flaw in Marx’s theory – value is not determined by the amount of work put into a product, it is determined by the law of supply and demand.
As for why Marx didn’t include the law of supply and demand in his theory, well he was trying to come up with an economic system based on the value of the ordinary working man. Unfortunately, as much as Marx would have liked it too, the market doesn’t value the ordinary working man, except insofar as his work plays into producing supply for which there is a demand.