Stephan,
The reason that we have the granularity of a file per method is to leverage git's history/tracking facilities on a per method basis.
At this level of granularity, you can move a method around in the package structure and git will maintain the method modification history because git treats these as a file rename and git is very good at tracking file renames ...
So you can use "blame" to view a line by line history of the method modifications ...
If one starts clumping methods into files, then you lose all method history when it moves from one clump to another ...
Dale