It's not. It's really not. There is no damn room.
It's the same with the MBA, it's the same with the Surface 4 and Surface. Hell, it's the same with every all-in-one now.
I'm trying to take you seriously here but you keep repeating this line that makes it sound like you have some deep and clever insight into a machine the press has barely touched, let alone teardown specialists. You're mad because it's not a full SSD on principle but you have no idea what the hot set size of the hybrid is. You've got a bone to pick with performance but Surface devices have always been forced to use custom configurations of hardware (e.g., every Surface Book has an unusual configuration of their video card that gives substantially better performance).
I think you're arguing based on theoretical specs and without knowing the domain. Unless you're an ex-Surface engineer with special insight, I don't see why anyone here should trust you about this given how you insist this is all stock parts and it's trivially verifiable that that's not the case.
I'm not arguing for abstract systems, but for that specific build that Microsoft has put together - in that form factor, a hybrid drive takes up too much space and they are not offering 3TB+ versions where the hybrid drive would excel.
I'm criticizing that specific assembly of parts.