A lot of the stuff we know now about optimizing wasn't really known then. It was new hardware and new technology for the most part. The hardware is what limited most games thought and the fact the system its self was a first shot at home 3d.
Lol wrong thread, you're right in a way, but Jet Moto 3 was Ming Lee's first PS1 project and he created the engine from scratch with absolutely no outside help. JM3 being later in the PS1's development didn't make developing the engine any easier, in fact, it made it way harder because he had to compete graphically with other games of that time.
He thought that JM2's engine was very basic, there was no lighting, the graphics weren't very detailed and it ran at 20 FPS. He originally wanted to make JM3 very similar to that, but with better optimization. I'm 99% sure that the game would've ran at 60 FPS had he replicated JM2's graphics in his own engine.