Because the wheels are designed to fit the chassis rather than other way round...
some cars use long wishbones, some short,they all have different hubs and base geometry,
On some cars the design wouldn't work if there had to be room for a hex adapter... mainly 2WD fronts so the axles/hubs can be designed to be as far out as possible, plus unsprung weight could be an issue.
they run different offsets as part of design so just bolting on any old wheels because they have a similar hex fitting is not advised as it could cause handling problems.
A lot of it also comes from American cars running imperial hardware.
You have to buy wheels as consumables either way so its not really a major issue, a headache when swapping brands but they have a used value.
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