Because game theory/prisoners dilemma/the red black game (whatever you want to call it) suggests you would get a better outcome otherwise.
if you ruin your only competitor, the antitrust folks will come after you
Nah, they still can come after you if you prevent new firms to enter the market. (But if you just do the usual regulatory capture the gov. will be just happy for you, as you gave them a lot of busywork for nothing.)