The problem is that short term selfishness often doesn't lead to long term success. If finally everybody came to the conclusion that it would be much more easy to upvote ones own comments or articles only, the quality of articles would decrease rapidly, new users would be completely frustrated (even more than already now) and the steem price would decrease as well. As long as only few accounts are doing that it may work very well for them - if all are doing it that's the END of Steemit.