[Cyberdog]

Because the Switch is running Factorio on top of a bit of code optimized for running games. A PC is running Factorio on top of a full-featured general-purpose OS, and most PC users are going to be running other things concurrently; web browsers, Electron messaging apps, malware, etc.

Of course, being able to target a single hardware platform with very minor differences like the Switch also means the ability to make optimizations to that specific platform without having to care about possibly breaking compatibility with others. It'd be like being able to optimize your game for people with Nvidia cards and completely ignore those with ATI, Intel, or any other brands on the PC.