Suppose we imagine a fantastical world where no one is able to trade, and a second fantastical world where nobody is able to labor.
In the first world, people would still be able to get stuff done, alter their surroundings, and invent new things - they'd just have to do it each for themselves. In the second world, there would be nothing to trade.
How can the process of exchange value be equal to labor value when it can only exist with the prerequisite of labor?