Well a lot of people sign up to check what it's all about, post something with more chance than not that it passes by unnoticed and give up after some time.
There's also a fact that not everyone is a great story teller, some people can post loads of bogus TA and deserve too much or make a bot votebuy scheme but most of the time ordinary users tend to get things said and done after a while and a writer's block occurs.
Curating the avalanche on posts that are mildly interesting to a user isn't that rewarding and can only get people being occupied that much either.
So other apps (even games) running on steem can/could be a blessing for retainment.