Quote from whitster:
"Sellers also equal buyers in the stock market. "
but that is not relevant.
NET longs = net shorts in the options/futures markets. the sum total = 0
net longs <> net shorts in the stock market. hence, the sum is not (necessarily) zero. therefore, it is not zero sum.
there are several other reasons to explain why futures/options are zero sum and stocks are not.
this is the most basic.
you CANNOT have a long futures contract, without somebody being on the opposite side. these contracts are AGREEMENTS. that is all.
you CAN have a long stock holding without somebody being short the same stock holding.
they are differently structured markets.