Demo is a mystery. Until the walls come down, you never know what you are dealing with. You never know is where the plumbing pipes run.
See the white pipes along the bottom and the right? Those are plumbing pipes which are expensive to move. Sometimes it’s cost prohibitive to move them.
What luck here! The pipes low enough and to the right enough to recess the medicine cabinet. Any space you can grab in a small bathroom is a feat.