They aren't, but the radiation passing through them causes single-bit 'style' errors because it's basically handing electrons to whatever it hits and thus flipping bits from 0 to 1. You can't even build a heavy lead shield around an existing CPU (which for space is dumb anyway 'cause you're paying about $40,000 per kilo to get stuff into orbit)... any spike in radiation that overcomes this will lead will junk your data. Same to all other bits of the machine. Drives have to be Faraday caged but most of the time it's better to just transmit them instantly down to earth.
I know a bit about the DoD microprocessor in satellites. It makes ancient look new, and I think the fastest they ever made was like 150Mhz, but the large process node means that it takes more electrical energy to disrupt them (just as a new 10TB HDD needs a weaker magnet to wipe as the individual 1s and 0s take a lower amount of space on the disk, and are designed to just need less power per bit in the first place).
So you have to make a CPU with the equivalent of RAM ECC on all it's processing units. Then you have to ECC the ECC lol.