[aidenn0]

While the kernel interfaces aren't exactly wildly disparate, they are perhaps surprisingly diverse (and coupled to kernel implementation details) for something as boring as a filesystem. At least some of this is for performance reasons. Accessing offsets into kernel structures is usually going to be faster than copying the data to use on your own.

On top of that, since there is no single standard interface in the kernel, kernel maintainers are skeptical of adding shims to the mainline kernel. The Linux kernel developers are perhaps the most famous for being vocal about this, but they are not unique.

That being said, there are portable file system implementations that use FUSE. See e.g. NTFS-3g[1]

1: https://en.wikipedia.org/wiki/NTFS-3G