Well now you see the two arguments. If they are people, how is that squared with “all men are created equal?”
If all men are created equal, how can one man be the property of another?
From one perspective, counting slaves leads to over-representation, from the other to under-representation. Which is how they came to compromise on 3/5. Admittedly a rather odd number.
True. But the point is, not counting them fully was a political calculation designed to marginalize the Southern states. Government policies developed in Congress tended to work to the advantage of the industrialized Northern states, encouraging manufacturing and more industrialization. The Southern states became simply a source of raw material to feed the machines. The North agreed to keep the agricultural economy of the Southern states that was designed around and dependent on slave labor as long as they needed to commodities it was producing.
That is the question you need to ask Thomas Jefferson who mischievously introduced this dichotomy into the American conscience.
Obviously no one agreed with him at the time, nor did he himself apply this idea to his own life.