Because insurance people like to confuse the crap out of everyone!
Truth is because at the origin of insurance there were two types: fire (which covered buildings for fire damage only) and marine (which covered ships). So in the beginning, real property was covered by fire insurance and personal property was covered by marine insurance. As insurance coverages grew, the terms such as homeowner, auto, etc. were coined to clearly market package policies that covered personal property, liability, and sometimes real property in a cookie cutter manner for masses of people with “normal” exposures. The policies that weren’t heavily marketed or that weren’t written en masse tended to retain their antiquated names. So even now a Commercial Fire policy usually covers way more than fire.
So, going back to the original marine (ship) insurance, that was the mobile personal property branch of insurance. When people started insuring other types of mobile personal property it more fit with the marine insurance than fire (real property) insurance so it split into Ocean Marine (mainly hull insurance) and the oxymoronic Inland Marine (personal property insurance on stuff other than ocean going vessels). Much of the personal property that doesn’t fit on a cookie cutter package policy such as a homeowner policy still goes on an Inland Marine. So now if you have a $100k Rolex or a herd of cattle or a construction crane or portable tools used in business or grain auger or tractor or baseball card collection, you put it on an Inland Marine policy.
Non-ocean going boats, such as something you can tow behind a pickup, might be on a boatowner policy or Inland Marine; the one time “Inland Marine” sort of makes sense.
If you’ve read this far, I am both surprised and impressed.