Because they regard themselves as a consumer technology company.
When you are consumer tech company, you can dictate your prices. Large enterprises want discounts. Apple doesn't discount.
When you are a consumer tech company, you don't have to release technology roadmaps. Large enterprises want roadmaps.
When you are a consumer tech company, you don't customize your product for an individual company or user. Large enterprises want customizations that work they way they want to work (good, bad, indifferent).
Apples doesn't consider itself an enterprise technology company because being an enterprise technology company goes against everything that makes Apple profitable: no discounting, closely held information and no customization.
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