If you look at it from the point if the economy, a handy man is a easily achievable job, meaning the number of handy man is large. Since there are so many of them, there is a large competition between them, lowering the value of there pay. What they do it specialized but not special. Some who differentiate themselves do make a lot more money. On the other hand a PHD is very difficult to achieve and a much smaller portion of the population receive on. On top of that a PHD makes you specialized on a topic, potentially making you a unique source of information which is worth a lot.
You are arguing for supply and demand setting salaries. But there is plenty of evidence in the economic literature which finds that is not usually true in real world markets (in fact, it turn out to be really hard to find any market which obeys such a simplistic model. And even when it does, it is usually easy to find scenarios where that same market doesn't follow supply and demand.) So supply and demand is mostly, in practice, just a contributing factor - and should be recognized as such.
