Hell, if you want to do that, you can do it. But there's no reason to want to do that. Partitions are an implementation detail of the filesystem. You shouldn't care whether /home is on the same partition as / or not.
If you read about the semantics of mounting a filesystem, particularly with regard to bind mounts, you realise that this is poorly abstracted. Mounting a filesystem and then binding it to a namespace are two separate actions. There is no API for the former.
POSIX added openat and related calls that operate relative to a specified directory descriptor, rather than the root of the mounted directory hierarchy.
It would also be possible to take this a step further and have calls which work with a filesystem descriptor and work on a filesystem independently of the mounted filesystem hierarchy. If it was possible to mount a filesystem without binding it (getting a filesystem descriptor), and then separately bind that filesystem into the mounted filesystem hierarchy using the descriptor. This would enable the direct use of a filesytem without it being bound, and that would allow for example private use of a filesystem by a process without it being globally visible, while might be useful for transient access to storage media without any potential for races (nothing else could open files or have a CWD in the filesystem).
I'll acknowledge that this is not strictly necessary, but it is an area where the underlying design is not well abstracted and is inflexible--sophisticated use of bind mounts is difficult, and the mount(8) dance to use them is terrible, all because it has to work around the lack of separation between mounting and binding.
Well, you could still add mounts – but while we currently can refer to devices by UUID, we can’t access their content by UUID, requiring ugly hacks such as sytemd’s automounting to /run/media/USERNAME/uuid/.
Hell, if you want to do that, you can do it. But there's no reason to want to do that. Partitions are an implementation detail of the filesystem. You shouldn't care whether /home is on the same partition as / or not.