In addition to the factors named by sibling comments, which I largely agree with, there is also the rise of short form entertainment on these platforms.

In 2004, social media was mostly text, images and low-fidelity game experiences like Mafia Wars. Compare to a bottomless scroll of immediate-attention-hook optimized, algorithmically targeted video content found on TikTok / Instagram.

The social behaviors got zombified out of the audience.