I'm not an expert (this is not my industry, so please correct me if I'm wrong) but I imagine it has to do something with the initial cost of entering the position.

With a futures position you may end up losing more than you put in (your initial margin) but assuming you went long a stock without using margin, the most you could lose would be your initial investment.

But perhaps it's not a fair comparison. I would say that buying 100K of stocks on margin is similar in risk to having a futures contract with a similar value, all other things equal.

Also, with forwards (not exchange traded) there would be counterparty risk if no clearing house was involved.