The reason Pong is not in Mame is that it can't be emulated. There is no software in pong to emulate, which is what Mame does. Pong was all done in hardware.
It looks like Pong was in mame, but it was removed becuase it was technically a simulation of Pong, not an emulation of Pong.
as Superdude said, Pitfall was an Activision release for the Atari 2600. There never was an arcade game called Pitfall.