The Southwest flight announced that they are ready after presumably holding short of the runway to do exactly what you just said. ATC also informed them that there was a 767 3 miles out right after reading the take off clearance. The Southwest crew read back the take off clearance, so presumably they also heard the bit about the other plane. Even without being instructed to expedite the departure, they should at least have realized that they don't have a ton of time on the runway.

More will surely be revealed in the follow up investigation and right now it's all armchair piloting. But I too am a little surprised the Southwest flight took its sweet time to do anything and was seemingly unaware of what was going despite being explicitly informed.

Totally agree we should wait for the investigation. It’s going to be very interesting.

To your point though, we also don’t know what was going on in the cockpit of the 737. Even if they were aware of how close the FedEx jet was, it’s entirely possible something went wrong that caused them to delay the takeoff roll (assuming it was delayed).

There are plenty of things that could have gone wrong on the ground. Separating traffic is the ATCs job and clearly there was a loss of separation. Very hard for me to see how this could be Southwest’s problem particularly since they weren’t yet airborne.