Pac-Man was included later, as I understand it, as a direct response to the Colecovision using Donkey Kong as a pack-in
When the 5200 was released, Breakout was the game, and they were all four-port.
Later, Atari found a fix for the four ports (to be compatible with the vcs adapter). They fixed and released the remaining 4-port back-stock. However, they had started using Pac-Man as the pack in game by then.
Damm, that means I had a rare atari. I wish we hadn't given it away.