I wonder the same thing. I just brought up Jar of Flies, and while it sounds a little dated, it doesn't sound ancient.
I blame record labels and radio stations, too. They still exist. I blame the system for the popularity of Nickelback a few years ago.
And it's funny you bring up Lady Gaga. To paraphrase an MTV VJ's description of Stone Temple Pilots: what doesn't she sound like? A lot of Pharrell WIlliams' stuff sounds like you could throw it at a DJ about 1982 and he would have given Michael Jackson some serious competition.
This wouldn't have seemed too out of place in 1986:
But yeah, I'm a little sad that music seems to have changed so little over the last 20 years. I mean, look at the Beatles: Please Please Me and Abby Road are only 6 years apart.