I don't think it's logical to assume that there is one singular root cause for the symptoms of ADHD (or any other symptom based diagnosis). There are likely a number of very different distal causes, although, because they present with the same symptoms, they may involve some common mechanisms. ADHD itself is defined as its cluster of symptoms, and doesn't imply or assume any singular underlying mechanism.
For example, streptococcus infection seems to be one (relatively rare) cause of ADHD.
As our medical understanding evolves, I think a lot of conditions currently based on symptoms will be found to be a cluster of different underlying mechanisms, and may eventually be divided into different conditions with different treatment approaches.
In reality, virtually all medical conditions are symptom based. Our knowledge of biology is very primitive and incomplete, so even where we think we understand parts of the mechanism of a condition, we are probably still missing much of it, and our perspectives on these change. In that sense, it's more clinically useful to stick with symptoms which we can observe directly, and not focus on ever-changing and incomplete theories about what might cause them.