Katy Perry might has "equaled Michael Jackson in terms of number of hits in the Billboard Hot 100" but those hits now came with much less sales than in Michael Jackson's era.
Ever worse, at MJ's time, if you had a hit, everybody heard it -- the channels were few and not that fragmented. Nowadays you can be Katy Perry, have a #1 hit, and half of the country wouldn't even know it, much less know how to sing it.
The cultural impact of Katy Perry is thus reduced to some teenage (or early teenage) kids, and tabloid gossip people reads and forgets that very minute.
And of course now there are 100 other things for a young person to do, between gaming, mobile, social, internet, anime, etc -- back then music was much more crucial.
>If you think these aren't as significant as [fill in your favourite act from decades past] then you're not the right demographic.
That's the thing though. You didn't have to be in the right demographic to think the Beatles significant. And even in the same demographic, you can have like 90% of people not giving a shit about Rihanna, whereas you couldn't for the Beatles.