They make up 70% of the desktop OS market.

Which is why Microsoft has been the target of multiple antitrust suits over the years.

Apple makes up less than 30% of the smartphone OS market share

iPhones command more than 50% in the US. In fact, iOS accounts for 67% of all app spending. The latter is what legislators care most about.

It should be noted that antitrust laws are broad, and don’t have any arbitrary market share requirement to trigger. Players can distort markets in many ways, directly and indirectly.