From this 2001 article:
Eric Horvitz... feels bad about [Microsoft Office's Clippy]... many people regard the paperclip as annoyingly over-enthusiastic, since it appears without warning and gets in the way.
To be fair, that is not Dr Horvitz's fault. Originally, he programmed the paperclip to use Bayesian decision-making techniques both to determine when to pop up, and to decide what advice to offer...
The paperclip's problem is that the algorithm... that determined when it should appear was deemed too cautious. To make the feature more prominent, a cruder non-Bayesian algorithm was substituted in the final product, so the paperclip would pop up more often.
Ever since, Dr Horvitz has wondered whether he should have fought harder to keep the original algorithm.
I, at least, found this amusing.
My first assumption is that almost everything you post is seen as (at least somewhat) valuable (for almost every post #upvotes > #downvotes), so the net karma you get is mostly based on throughput. More readers, more votes. More votes, more karma.
Second, useful posts do not only take time to write, they take time to read as well. And my guess is that most of us don't like to vote on thoughtful articles before we have read them. So for funny posts we can quickly make the judgement on how to vote, but for longer posts it takes time.
Decision fatigue may also play a role (after studying something complex the extra decision of whether to vote on it feels like work so we skip it). People may also print more valuable texts, or save them for later, making it easy to forget to vote.
The effect is much more evident on other karma based sites. Snarky one-liners and obvious puns are karma magnets. LessWrong uses the same system and is visited by the same species and therefore suffers from the same problems, just to a lesser extent.
Also, sometimes an apparently well-researched article turns out to be based on only a superficial understanding of the topic (e.g. only having skimmed the abstracts) and mis-represents the cited material, and this is sometimes revealed on "cross-examination" in the comments.