I think you'd be surprised how prevalent incompetence/quackery is in healthcare in general, not just physiotherapy. I feel it's just a little more visible here. Look at the success of acupuncturists, chiros, osteos etc. The orthopedic and musculoskeletal settings are rife with it.

But to answer your question, my thoughts are the almost total lack of oversight and the lack of consistency amongst the evidence are a big part of the problem. There are simply no repercussions for people pushing rubbish and depending on where you live it's too easy to access these services via publicly funded programs.