The fonts look a bit different from the Microsoft and Apple ones, but they were likely sourced from a different vendor (URW?), so there's an uncanny-valley aspect to it these days when "Times" means "Times New Roman".
I wonder if part of it was that when the earlier GUIs, like GEM and the Workbench, were architected before desktop publishing became the undeniable "killer app" for a GUI. They figured "here's some basic fonts to render the UI" and punted to the application software to handle anything else. Even with some of the early GUI word processors (GEM Write, for example), full WYSIWYG typography might have been above the expected target, considering the machine might be paired with a daisy-wheel or 9-pin dot-matrix printer.
Of course, all typography of the era looks a little odd by modern standards, running on a 640x480 screen with no antialiasing.