Two reasons why ...
If Apple is so confident of the superiority of it's designs they should have no worry about the acolytes looking elsewhere. Any organisation that not only denies but enforces it's afficionado's from trying alternatives is
1) Supremely arrogant.. "We the already enlightened have seen the truth and can save you, join us and do as we say"
2) Running scared "We the already enlightened know we are selling you a crock of crap so we can't allow you to go elsewhere."
It smacks of at least the Wizard of OZ comment "Pay no attention to that man behind the curtain." or at worst the sinister genuine cults <insert_crackpot_religion_name_here> . A shame really because once you strip away the marketing guff and huge mark ups, Apple kit is well made and well thought out.