Because it's a random third party filesystem adaptor module.

Why would it be "included by default"?

Most of the Unix utilities that are included were written by third parties. They're "included by default" because they're useful and take up a negligible amount of space.

They're included by default because they are standard UNIX userland.

FUSE is nowhere near that.

FUSE is newer than netcat or bzip2. That doesn't mean it isn't expected as standard. Linux, BSD and others all have it.

Where is FUSE in the POSIX or SUS standard?

A machine without FUSE is a toy

Because it's necessary for a load of use-cases, and installing a *kernel* extension for it is terrible for deployment

MacFUSE isn’t even open source anymore, so it would likely involve Apple forking a version prior to the license change and maintaining that themselves.

They have their own APIs for it but seem to have assessed that the only mainstream use case is cloud sync, which they always provide a better API for.