It's crazy how much work went into those old games. I have a feeling those programmers weren't even paid that well considering how few people owned computers back then (so the market can't have been large).
There were programmers in other divisions making royalties off of their games. Tod Frye famously got $700K or so for his terrible version of 2600 Pac-Man (it was terrible not because he was a bad programmer, but because marketing decided that 2K of ROM had to be enough, and he was smart enough to pull off a miracle . . . of sorts).
Also, the OP apparently doesn't know how to unroll loops, which is the first thing you do to your game's hot spots. (Never had to resort to self-modifying code).