I didn't say it was easy. redis is full of reinventing the wheel in the name of improved performance. This is something developers often do, but are often told it's pointless, just use built-ins.
Salvatore has found a niche where he can see these optimized implementations to their end. I would imagine there is a certain level of satisfaction that comes from thoroughly researching a topic, implementing it as best you can, and deploying it to the public you're popular with. It probably helps having firm control of the project with less red tape.
It's like a comparison between a craft woodworker and someone who would like to play around with the concepts, but doesn't have the tools, time or know how, or they're told there is no point in building your own furniture from scratch, so they are instead building Ikea furniture.
Maybe it's a case of "developer interest dissonance". Technical people digging technical problems (cache hit issues) versus business logic... which might be why they "got into computers ".
Low-level high-perf programs are extremely demanding and require a good deal of care and persistence.
"The simplistic low-level nature of redis probably helps avoid fatigue."
Yea I'm going to bet you never wrote anything where microseconds or L1 cache misses mattered.