Maybe part of the issue why people don't develop further: too many of us enable others in the name of upholding some social convention or given expectation of who we should be, and become co-dependent in unhealthy relationships. We also don't have alot of guidance on what is healthy behavior anymore, especially in modern society with its libertarian notions of personal freedom and fulfillment, which are oriented towards maximizing self-expression and material achievement only (basically, one one personality type has a realistic chance to thrive under this default mindset, everybody else is almost guaranteed a certain level of dysfunction).
This is where studying the Enneagram has been helpful for me as well. I'm a type 4 with a wing of 5, the Romantic, but also an Investigator, to a slightly lesser degree. Years ago, I was a people-pleaser and deffered to authority figures, and I thought that was the "best" way to be a Christian. I am a highly sensitive person, and I developed fibromyalgia as I slowly took on the worries, concerns, and even opinions and customs of others, degenerating towards the type 2 (the Helper). I should have been focused on being more like a Type 1, the Reformer (oriented towards truth and moral principles), and a type 8, the Challenger (more self-assertive). Since that time, I've been learning to take care of myself, to be unconventional if necessary (as a Romantic it's basically who I am, anyways), and my fibromyalga has mostly gone away over the years.