Because no one in Northern California, probably north of Santa Barbra calls it PCH.
Up here people call it "Highway 1" or "the 1" but never PCH. It also goes by Cabrillo High, but I've never heard anyone call it that.
I call it Cabrillo Highway all the time :P
Easier to call it 1 though.
US-101 is the Pacific Coast Highway north past Mendocino County, However I was not aware they call CA-1 that down in L.A area.
I call it PCH” and I live in Georgia.