JetBlue doesn't overbook and they have similar load factors to the big US legacy carriers, so it's clearly possible to not overbook

They also don't care that it eventually runs the company in the ground because they'll be gone when the thing finally breaks.
Short term gains for long-term failure is a trade they'll make without thinking twice.

Do you not understand that the supply (number of planes) is constant throughout the year, while the demand (number of fliers) fluctuates? If the supply enough planes for everyone to fly at peak times there's going to be tooons of empty seats throughout the rest of the year. They'd then need to raise everyone's ticket prices to pay for those extra empty planes, and people just aren't willing to pay the extra price for the convenience.