i've built PC's on carpet/beds/bedsheets for years without any issue.
TBH, its not the static that kills them - its random dust and debris falling into the slots, or solder points catching on carpet and getting damaged. seriously, you'd need rubbing slippers on carpet to get this to be a problem - if your PC is in a case and you are touching that case, you and the PC have the same electrical charge - there is no discharge, there is no damage.
"static electricity killed your PC" is computer techie lingo for "i have no idea why its broken, but i'll charge you $150 for it anyway"