If this syndet bar works the way you want it to, there's no reason to look further. But don't assume what works well for you will work well for everyone else.
There are a number of reasons why many 'poo bars have a longer ingredients list.
You'll often see two or more cleansers used in many shampoo bars because it is a demonstrated fact that a well-chosen blend of two or more cleansers will be milder than just one cleanser alone. This is also true for cleansers used for regular bathing, not just for washing hair.
Even lye-based soap can be blended with one or more syndets to make a mild "combo bar." Check out the ingredients in Dove cleansing bars for a popular example of a combo soap-and-syndet cleanser.
There are many reasons why non-cleanser ingredients are used -- they can further modify how the product cleanses the hair and they can also improve the feel of the hair or skin after cleansing. The coconut oil you mentioned is there for this reason -- it reduces the cleaning power of the cleanser and perhaps adds a bit of conditioning to the hair after the lather is rinsed off.
The problem with coconut oil (or any other fat) is that more than a touch of fat in a shampoo bar can weigh down fine hair and even leave the hair feeling a bit greasy. So there are other ingredients that can be used instead to provide a light amount of conditioning for fine hair without adding heaviness or greasiness.
Still other ingredients can be included to soothe itchy scalp, make the hair easier to comb when wet, reduce frizziness and static cling, smooth and strengthen the hair shaft, etc.
And some ingredients add "sales appeal" which is important in this increasingly crowded market.
In addition to all that, there should also be a preservative included in syndet bars to inhibit microbial growth on the wet surface of the bar.
