Are you saying it’s strange to require an adapter to add storage to a computer? Because that’s been the case ever since computers have existed.
On what planet? It's not even true on planet Apple - the original Mac Pro tower was a particular breeze to add up to 3 extra HDs to with no extras required - 3 empty almost tool-free HD sleds included and didn't even need any cables. Pretty sure all the preceding "Pro" towers going back to the blue & white G3 had bays for internal drives.
...and then there's just about every PC desktop/tower in the universe that isn't a laptop or small-form-factor - which typically have empty drive bays that need at worst a (bog standard) SATA cable and a few (standard) screws to add a drive. Then, a lot of modern motherboards have spare M.2. slots for (bog standard) PCIe SSD modules that you can install yourself.
So, yes, the Mac Pro is very, very strange for a pro tower workstation in not having any fittings for non-proprietary internal storage expansion. The area where the j2O (or whatever) goes is clearly designed for that, but Apple decided to leave out a few bucks worth of aluminium needed to actually use it...