The three reasons you have pointed are very valid. These are the top most reasons for leaving this amazing platform.
There are a lot of people who have very small SP and low reputation score but they create quality content. Their blogs don't get enough attention so they are frustrated and leave platform.
Some people come here to gain quick money but that is not possible unless you persevere and create quality content and make strong bonds with people. They don't want to wait and desperately quit steemit. Doing consistent efforts and staying on track is the key to be able to get success on this forum.
One more reason is that not every one is blessed with creativity. Some people cannot find their niche to make quality content so they only try to curate and comment on others' blogs. Some times they don't find good response, dishearten and leave the community.
As you have stated patience is the key to get success. Thank you @humanearl for a thoughtful blog.
I've been asked that question many times. "What should I talk about in my writings or videos?" I simply say talk about what you really enjoy talking about because there will be others who enjoy it as well.