This started happening sometime in the mid-90s, but it continues to this day.
Millions of young people embrace the music of their grandparents' generation to stake their claim to inviduality in a world where every form of musical rebellion and individuality has been commodified and sold back to them. Including, eventually, lounge. When Robbie Williams is doing covers of Sinatra et al, it's time to move on, you faux hipster nitwits.
Oh, they'll claim, "no, we just like it because it's great music." Tony Bennett is great music? What? Really?
It's rubbish. It's sedate, anodyne, empty rubbish for the privileged set circa 40 years ago. The very generation that's occupying positions of power that deprive them of jobs through cronyism, send them to fight illegitimate wars, plunder the environment, enact laws that send their contemporaries to prison for taking the drugs they like.
Yay for marketing.