Because it would be, if my suspicion proves to be the case, the point where adherence to the FHS tips over from being the norm to the exception. MacOS, Windows

Because it would be, if my suspicion proves to be the case, the point where adherence to the FHS tips over from being the norm to the exception. MacOS, Windows, and who Arch users are have nothing to do with it. How many distribution creators have made each of the various choices, does. Arch's decision to use systemd had some influence (in both directions, c.f. Hyperbola Linux). Arch's decision to abandon the FHS in favour of the systemd file-hierarchy may well do likewise.