"average attention span" is pretty meaningless. It's very much task-dependent. How much attention we apply to a task will vary depending on what the task demand is ... the idea that there's a typical length of time for which people can pay attention to even that one task has also been debunked.

I do agree that some people here on steemit pay less attention to the post and focus more on what they can earn rather than learn. They tries to maximize their time on each post, sometimes speed reading it to the point of only reading the title. haha