There is still risk. Even if you never lose money on a trade, your firm can lose money. You're paying fixed costs such as colocation. It's conceivable that all your trades are profitable, yet the sum of all those profits is lower than your fixed costs.

That becomes more likely for any given firm as the competition increases. Spreads get smaller; there's more competition for any given trading opportunity. For each firm, that can mean declining profits per trade and declining numbers of trades made. When the product of those two numbers gets too low, you're in the red.