A classic result in psychology can help understand why Steemit is addictive. It has been shown that rats, when given a lever that provides food every 3 minutes, don't become addict: they learn to push the lever every 3 minutes. However, if they have a lever that provides food at an average of every 3 minutes, but with a big variance (for example 1 time it will be after 5 minutes, the next time after 30 seconds, etc), rats become addict and continuously press that lever!

Keyword here is "variable reward". You don't know in advance what you'll get, so you check all the time. It's the same with facebook which is also addictive (the variable rewards being the notifications), but money is of course an even better reward than social recognition.