Because it’s in the inspector with everything else. Judging from other apps, it’s designed to resemble the same experience on macOS and iOS versus having two designs and two learning curves. Omni has done a great job with this app.The functions and locations will be the same. It consolidates all options to one window versus having options scattered throughout the app. You will find this design decision in many other apps besides Omni’s apps. Adobe is one large example. Inspectors have been around for a while.
You can right-click the top toolbar and modify it by adding the font option as a shortcut. Modifying the toolbars have been around for years for macOS apps. I’m not sure about 25 years, but definitely over a decade.
For all apps, I recommend learning them from head to toe before judging so quickly. OmniOutliner is very powerful with a ton of options you won’t find in many apps.