The answer is because they are all the same and therefore risk you going elsewhere.
Think about this point. If you looked at all the institutions offering wealth management services, whilst everyone wants to be distinguished from the rest, exactly how are they that different?
Usain Bolt we all know was a great runner. There were lots of runners but only one Usain Bolt. He was recognised as the best runner.
Is there any wealth manager or private bank that can justifiably prove they are head and shoulders above their competitors? The answer is no. If there was, everyone would go to the best.
If there is no one firm that is the best; there are nevertheless thousands who will tell the story you want to hear. Ever thought it strange that whatever you want is what you are offered? Is there a chance your ego is being stroked?