I blame hustle culture and self-improvement gurus. They have hammered the message "if you're not making yourself more marketable, or grinding to make money, then you're failing".
I feel this might be the biggest factor. Hobbies were one of major ways to deal with boredom and mundaneness of life. Nowadays, there's endless content online, shows, video games, restaurants, concerts, travel destinations, gadgets that people can consume and basically never feel bored.