Because we miss the usenet newsgroups, of course.
I think a large part of the success of this particular forum is the level of discussion, in topic selection, and in the standards of postings.
Basically, most of us seem to be folks who are filled with curiosity, and also crammed full of information that we like to share.
The culture of proper writing, understanding of sound “argument” techniques, and a basic respect for fellow dopers attract many of us.
There are also different fora for different interests. I personally almost never go to the entertainment oriented sections, preferring General Questions, Great Debates, Mindless postings, and In My Humble Opinion to others. Still, there are other dopers that do not share my tastes, and that is OK.
One factor that is rarely mentioned is the maturity of discussions. While delighting in the deliberately whimsical and punnish contributions, it seems to me that I can have actual adult conversations here, without enduring the rolling eyes of my real life friends; while wonderful people, they are hard pressed to deal with my unusual interests and opinions.
So what is not to like? This is a fun place to play!
This shows what the OP is missing.
On most forums, it’s not random people answering random people.
It’s people with varying degrees of interest, experience and knowledge of a given topic answering someone less knowledgeable about the topic.
Someone curious about the topic can ask a question that even google can’t answer. The members of the forum get to have the warm fuzzy feeling of helping out their fellow man and potentially finding a kindred spirit.
It’s pretty rare that a question doesn’t get a reply. It’s way more often the case that the questioner never replies in the thread they created so the answerers never know if the OP even read the answers.
I came back from bush Alaska in July 1997 and discovered forums and was very soon my question: Why do people use forums?
I was interested in gardening and found a forum, remnants sill around, called gardenweb .com. I soon noticed that if the title line were a question which could be answered yes/no then sooner or later both answers would be posted. [ I bookmarked such titles that accrued 10 or more replies and when I had 100 bookmarked I checked them all. Sooner or later 92% got both a yes and no answer.] Some had ambiguous backgrounds in the street legends e.g. Is rhubarb poisonous? Others were just opinions i.e. is organic beneficial. Some were just wrong i.e. does Burpee sell carrot seed? etc. 92% nonetheless! So, Why do people use forums?