Some of this may be Google shipping its executive incentives (as in “Don’t ship the org chart”).
Another example: Google Street View is available on the web - unless you use an iPhone and enter the address directly on maps.google.com instead of google.com. On the maps hostname with an iPhone, clicking the Street View preview just shows a nag screen that it’s only available in the Google Maps app. That’s not true, so I assume that app installs is some group’s KPI.
And conversely Google Earth Pro can view street view imagery, too, but only the current version – historic street view is only available via Google Maps through the browser.
When you're talking about Google products, fragmentation comes built-in as expected. There is no reason for 3 versions of maps to exist within Google ecosystem but I don't think they really care
For some reason historical satellite photos are available in Google Earth Pro (which is free) but not on maps.google.com or earth.google.com