Because people don't know what pop music actually is. They don't realize that not only does it include Just Bieber and Lady Gaga, but also Jonsi and Grimes, Billie Holiday and Jacques Brel, Francoise Hardy and Marie Laforet, Echo & the Bunnymen and Erasure, Kate Bush and Peter Gabriel, etc... Even very avante garde/underground artists like the Cocteau Twins were regarded as pop (namely "dream-pop").
My old rant: All rock IS pop. "Pop music" was coined in the 1950s to describe popular artists of the time, including (but not limited to) the pioneers of rock n roll (Elvis, Chuck Berry, Little Richard, Eddie Cochran, etc...). It's not really until the mid/late '70s (when it went into the polarization of hard rock vs. disco) that rock was seen as something other than "pop" - it was just one branch of pop music.
And if someone thinks pop music has "meaningless" lyrics (as if rock often doesn't... please tell me the deep meaning behind Poison's "I Want Action")... Howabout Jacques Brel's "Jef", about a guy damning a woman for being a "whore" (word actually used, in French) and considering suicide/the narrator talking him out of it, but also admitting, "yeah, you actually probably should kill yourself". Or for a more modern example, Jonsi's "Go Do", about embracing death, living for those who have died, and seeing the potential and beauty of everything as a way of carrying on, and "singing out" all of that. Hardly "meaningless".