[iLoveOncall]

I think it's because all of the low hanging fruits have been picked up already.

People used to make 30,000 reputation out of a single answer on an extremely basic topic like "how do I create a variable in X".

Now all the questions are either duplicates getting closed or hard questions on very specific topic, that, even if you can answer, will get you 10 reputation, and maybe 50 over the years.

On the other hand, the spam of stupid, badly formatted or undersearched questions is intense.

So StackOverflow has basically become a platform where contributors have nothing better to do than moderation, for which there is no reputation reward.

I know someone will tell me it's not all about reputation, but I don't believe it one second.

[listenallyall]

Very true. Further, most long-time contributors to the site have realized the rep points actually have zero value. Would you hire someone because they had a high SO score? Would you put your rep balance on a resume?