https://uproxx.com/music/2013/07/seven-nation-guitar-army-the-8-most-recognizable-guitar-riffs-of-the-2000s/Found this list searching for best known of the 2000s; the first two were Seven Nation Army (which definitely counts) and No One Knows by QOTS which I thought was a pretty good contender. So maybe some of the other riffs there might be good options (though it also has Miley Cyrus - Party In The USA in there, so I don't know..)
We all have our idea of things that we think are well known because lots of people seem to know them in our own experience... until we find out loads of people have never even heard of them. So I'm not sure if some of these are as famous as I might guess, but a few more ideas:
Foo Fighters - The Pretender or All My Life
The Strokes -
ReptilliaRed Hot Chilli Peppers - By The Way or Can't Stop
Rage Against The Machine - Killing In The Name (not that much more recent)
Green Day - American Idiot
The Foo Fighters and Red Hot Chilli Peppers I think are pretty well known... All My Life and Can't Stop are the ones that have the more memorable "classic riff"-style riffs imo but maybe other more well known songs will actually have the better known riffs. Linkin Park had some very well known songs but I'm not sure what would be the most recognisable riff from them - In The End or Numb I'd guess would be the most recognisable songs but it would probably be the choruses or piano / electronic riff people recognise most rather than a guitar riff so I'm not sure if they would count for this discussion. Something like Avenged Sevenfold - Nightmare comes to mind too, but I'm not sure how well known outside of rock / metal circles it would be.
As for why it's more difficult to pick out "classic riffs" that everyone knows from more recent times, I don't think it's because bands aren't producing good or memorable riffs, it is just because of the change in the music industry and pop culture in general. Rock is not as big in the mainstream as it once was, and the way people listen to music means that there are perhaps less things that "everyone has heard" because it is much easier to find ways to listen to the music that suits your own taste instead of only getting to hear "whatever's popular".