This could be a space-saving measure. A bathroom also needs to be used by other occupants of the house/apartment (children, guests staying over etc.) and if it's only available by passing through the master bedroom, it becomes a privacy issue.
As a consequence, you need to add more bathrooms (which requires additional space and money) or make the primary one accessible in a more public way (from the hallway).
That's a valid concern, I wouldn't want guests to have to move through my bedroom to access the toilet either. However I've seen lots of American apartments with a two door bathroom - one to the master bedroom, the other to the hallway. That definitely does make the most sense in my opinion.
Aside from this, even in spacious and expensive German flats or houses, this isn't common.
As a consequence, you need to add more bathrooms (which requires additional space and money) or make the primary one accessible in a more public way (from the hallway).
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