Because it is generally accepted that monopolies are bad and that not-monopolies are ok. It is much easier to sell the simple idea that "X is a monopoly, so spl

Because it is generally accepted that monopolies are bad and that not-monopolies are ok. It is much easier to sell the simple idea that "X is a monopoly, so split X", than to make a long case that "X does Y and Z which meet ABC definition of unfair use of market position, bla bla bla, so split X". Conversely, for a company in the wrong, it is easier to say "Of course monopolies are bad, but we are not a monopoly!".