All of them enjoy market domination in their respective areas of expertise. Their size makes them effective monopolies and their behavior has many times ruled anti-competitive. Competition between them is insufficient and they have been convicted of organizing in monopolistic manners before.

For consumers and the market as a whole it is more healthy if they are broken up into smaller companies.

Even if they weren't near monopolies in their respective fields, I think we should be wary of permitting the sort of concentration of power that arises from the existence of trillion dollar corporations.