How does a single dust particle interrupt a circuit? (I know nothing about this; it's interesting.)
https://www.ibm.com/docs/en/power9/0009-ESS?topic=floors-con...
I still imagine it's insanely overkill, but when you're paying millions of dollars to send something into orbit, you probably want to make sure it works well first time.
With regards to malfunctions, I imagine this risk would actually be better mitigated with (fail-safe) circuit redundancy. After all, radiation is going to be causing bit flips and all sorts of weirdness.
That's not to say redundancy is bad or should never be considered but it needs to be weighed (literally and figuratively) against every other part of the mission. Optimizing for mass and mission capability means manufacturing will get more expensive.
There is also the risk of short circuit on non-coated PCB, increased ESD risk. And beyond PCB, there are a lot of other risks mitigated by clean room environments: dust on sensors, dust on bearings or gears, dust on optical cameras... And any deposit of dust will float around everywhere once in orbit, especially after being shaken and vented during launch.
Could you make a satellite in your garage without a clean room environment? Yes sure, and you would even have a decent chance to have a working satellite once in orbit if you know what you are doing. Would you accept a 50% failure rate if you are flying a payload worth a few millions and need to spend a similar amount on launch cost? Probably not.