There was a very good discussion by the DOJ in the AA/US case of why the legacies do not compete against the others' fortress hubs.
If DL was to try such, UA could not compete by dropping prices there, as they would end up having to reduce prices on far more fares. So UA would retaliate in a DL hub.
The net result is a loss for both airlines. Thus it does not happen.
WN can compete, because they have no hubs.
US (pre-merger) could compete, because their hubs sucked.
But the big 3 will not compete.
Oh, DL did go into SEA. But AK is not all that big (and it would not surprise me if they are just dancing anyway).