Because Apple ran a series of commercials comparing Mac to “PC” to differentiate themselves from the rest of the market but didn’t acknowledge other OSs besides Windows in this series.
Those ads played everywhere and now everyone associates the term PC with Windows.
I could have my timeline off but the first time I heard PC to refer to windows specifically was around the time these commercials aired.
Honestly sometimes people just need to vent their frustrations and aren’t interested in immediately starting a new project to solve their problems. This applies to life in general not just Linux.