I think the problem here is that people are jumping from "git should be able to store binary assets" to "git should be able to store binary compilation objects."
You would never check a .o file into your SCM; an SCM, as the name implies, is for managing source files, not object files. A PSD or a DOCX is also a source file, despite being binary: they're representations of the work-in-progress itself, containing enough data to let you resume editing the document. A JPEG, on the other hand, is a compiled object—something you export from your image editor, not something you open and edit and save again. (Unless you're intentionally going for that recompressed-shitpost look, I suppose.)
When you pull down a source repo—a thing you edit—you should expect to get source. That applies to both your text/code assets, and your image/binary assets.
If, on the other hand, you need some assets to just sit there and be consumed by your project, then those aren't source, and so don't belong in your source repo. Those are likely dependencies, which can be resolved to (a triggered compilation of) the relevant source repo for those assets, or which can be resolved to a linear(!)-versioned binary package containing the compiled objects for those assets.
Which is all to say: PSDs, given an appropriate diff tool, could go in git. Final, "product" JPEGs, on the other hand? Those should be sitting in a gem (or equivalent), which was the tagged continuous-integration result of pointing a buildbot (exportbot?) at the relevant source repo full of PSDs. When you build your project, that gem gets pulled down (hopefully from your own private CDN) and suddenly you have some JPEGs, just like suddenly you have some native module .so files.
Someone could write an image editor where the project format is a directory with the source jpeg or png assets and a makefile which generates the resulting image with imagemagick (or other) calls. The makefile could be easily diffable and mergeable and people wouldn't touch the binary assets (or at least wouldn't expect to diff or merge them).
Brushes wouldn't be easy though. This could be handled by trivial svg or ps paths which would get rendered in the make process. This way even brush strokes could be diffable. I wounder if it would be possible to convert existing PSD files or Gimp project files into such a format.
You would never check a .o file into your SCM; an SCM, as the name implies, is for managing source files, not object files. A PSD or a DOCX is also a source file, despite being binary: they're representations of the work-in-progress itself, containing enough data to let you resume editing the document. A JPEG, on the other hand, is a compiled object—something you export from your image editor, not something you open and edit and save again. (Unless you're intentionally going for that recompressed-shitpost look, I suppose.)
When you pull down a source repo—a thing you edit—you should expect to get source. That applies to both your text/code assets, and your image/binary assets.
If, on the other hand, you need some assets to just sit there and be consumed by your project, then those aren't source, and so don't belong in your source repo. Those are likely dependencies, which can be resolved to (a triggered compilation of) the relevant source repo for those assets, or which can be resolved to a linear(!)-versioned binary package containing the compiled objects for those assets.
Which is all to say: PSDs, given an appropriate diff tool, could go in git. Final, "product" JPEGs, on the other hand? Those should be sitting in a gem (or equivalent), which was the tagged continuous-integration result of pointing a buildbot (exportbot?) at the relevant source repo full of PSDs. When you build your project, that gem gets pulled down (hopefully from your own private CDN) and suddenly you have some JPEGs, just like suddenly you have some native module .so files.