Well ignoring piracy and the typically low sales of PC games outside certain genres - the PC has a massive support cost, because everyone has a different system configuration - so there's far more bug testing and support training needed to be on the PC. Millions of different combinations of OS, CPU, Graphics, etc - all need to be considered
Combined with that, PC games traditionally sell at a lower price point (AC13 is half the price on PC compared to the consoles on Amazon, and $20 less at EB Games here) - so that for the increased support costs you get lower revenue (it's not 1:1, you don't have to pay the platform fees to be on PC, but that alone doesn't make up the price differential expected by the market).
Consoles have the benefit of being all the same (so lower support/testing costs) and they have a higher expected price point in the market - a premium to save the hassle of PC gaming and perhaps to some extent a built in hedge against the second hand market.
More 'sales' doesn't make PC editions profitable. Unless the first person buying the game on PC pays a million dollars for it, then it's a cost benefit issue.