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.