>several examples
JIS isn't relevant outside of Japan, though. Everyone cares about ISO (USA) and standards like RoHS (Europe), nobody even knows what a JIS is.
>there’s a practical reason for camera brands to have different mounts
The reason is irrelevant, it's a camera and a lens and the fact they are incompatible is inexcusable. Japanese ham radio takes this further; when I said Kenwood microphones don't work with ICOM radios I mean they could use the same number of wires and perhaps even the same connector but they keep the wiring positions proprietary.
So you want to use your Kenwood gear with ICOM? Or Yaesu? Well too fucking bad bud, either get out some wire cutters and a soldering iron or buy into one of the ecosystems like a good Nippon-jin.
Imagine if Logitech, Corsair, Steelseries, and Razor all used USB for their peripherals but the wiring is unique to each of them, and imagine if mobos brands like ASUS and MSI partnered with specific peripheral brands and only featured USB ports compatible with them alone.
That's the Japanese ham radio scene and yes it fucking sucks; and yes, they are getting eaten alive by cheap Chinese ham radio equipment that doesn't do that proprietary shit.
If ham radio is too niche for you, don't worry: It expands to stuff like household appliances too. If you buy a Panasonic microwave oven or fridge and it breaks down you bet your ass you will need to buy replacement parts and perhaps even the repair labor from Panasonic.
We all love to complain about walled gardens and planned obsolescence here in the Anglosphere, but Japan is several orders of magnitudes worse. They make Apple look like a paragon of standards compliance and interoperability.