Amazon and Google are favored because they have close to a monopoly in their respective core businesses, and there is not even a hint of a threat (I personally prefer duckduckgo to google search, but it might take a while for duckduckgo to catch up). Apple is the leader in the mobile device business, but it needs to keep reestablishing the leadership -- much less clear that they will keep succeeding for the next five years (say). Note that this is the bargain with the devil they made: they jacked up the prices, which generated their cash hoard, but also made it easier for people to compete with them, and the low end of the smartphone market was left completely open (now it is completely dominated by Android).
Monopoly for Amazon and Google. No way. Walmart is the real competitor for Amazon and Google could easily get competition if Apple was serious with Siri