The reason why is that they are two different regulatory authorities and areas, and there is no particular reason why they should do things the same way! Both systems have evolved over many years to suit the markets they serve. The American system is, like many of their ways, less regulated and bureaucratic than Europe, but then the employment system is far more risky and - until you become very senior in a major airline - less well paid than in Europe.
Neither system makes it easy to become a commercial pilot - and who thinks it should be easy? It's slightly cheaper and easier in the US, but the saving - in my opinion - isn't significant enough to give up the advantages of employment in Europe. Ask the 10,000 or more furloughed US commercial pilots where they'd rather be just now!
Scroggs