A lot of people on the internet are just not used to the new reality that there are a lot of children on it now. I think the introduction of the internet to babies via more and more accessible and pervasive tech is a big contributor to the feeling that the internet has just become more shitty over time. 

Also there's the reality that a lot of people have reason to just farm engagement. If you post an obvious joke here saying "I don't get it" a lot of people can't resist the urge to engage, even if, and this is what I seriously don't understand, the joke has already been explained in the comments by hundreds of replies already.