Earlier quoted context omitted.
For the same reason antitrust laws exist. Monopolies need to be prevented to maintain a healthy economy.
How is Apple a monopoly? They own a system and manage the rules around it, that is not a monopoly. Will you force car manufacturers to let you install any system in the car? Why can't I install any app on my car?
https://en.m.wikipedia.org/wiki/United_States_v._Paramount_P....