It isn't, the direct flight would just need the potential energy of the separation between the Mun and the spacecraft,  it is just harder to time things for a direct flight that's why we don't use it.

Because, the transfer orbit makes use of existing potential energy of the separation between the Mun and the spacecraft and needs only to add a little more energy as a result to get to the Mun's orbit.

Because, the transfer orbit makes use of existing kinetic energy of the spacecraft and needs only to add a little more energy as a result to get to the Mun's orbit.

It isn't, the direct flight would just need the potential energy of the separation between the Mun and the spacecraft, but it is not possible to fire the engines in the direction required.