Pong was written with zero lines of code

The game also has an interesting history:

"Prior to working at Atari, Alcorn [the engineer who created Pong] had no experience with video games. To acclimate Alcorn to creating games, Bushnell gave him a project secretly meant to be a warm-up exercise. ... The prototype impressed Bushnell and Dabney so much that they felt it could be a profitable product and decided to test its marketability."[1]

As to why Pong was implemented in hardware rather than software: the first Intel microprocessor, the 4-bit 4004, was released in 1971 and the 8-bit Intel 8008 was released in 1972.[2] Pong was created in 1972, so at the time microprocessor chips would have been expensive and esoteric components with a limited supply. (The 8008 chip cost $120 in 1972, which would be $725 in current dollars.) Meanwhile, minicomputers made of TTL chips were large and cost many thousands of dollars.

[1] https://en.wikipedia.org/wiki/Pong

[2] https://en.wikipedia.org/wiki/Microprocessor