Smartphones ruined the internet.
When you needed to sit at a computer, use a keyboard, and spend a bit of time to write out thoughtful content, the quality of the discussion and community on the internet was so much higher. With smartphones, they had to dumb down the UI, consolidate content and reactions into likes and emojis, and generally just had to deal with smaller, more unsubstantive and rage-baity comments.
Plus, there's just the undeniable fact that computer users are more tech-savvy than people who only know how to use a smartphone. Making the internet smartphone friendly opened the floodgates up to a sea of low-quality engagement.
Before smartphones, we would go outside, live our lives, then come home and see what happened on the internet that day. When the internet started following us around in our pockets on crippled devices that are designed for consumption and cheap-ass engagement, that's really when the internet went to shit.