I think there should be disincentives for any company to grow beyond a certain size. Huge companies provide a short-term benefit to the consumer but in the ling run they cause a lot of damage to innovation and the economy.
Anti-trust in the U.S. has not been super strongly enforced in recent decades (perhaps since the Ma Bell breakup and Glass Steagall before it was repealed?). But if the consumer is not being harmed, why get in the way of a company that has truly earned market dominance?
Presumably the anti-trust regulation exists to determine the consumer harm.
[1] https://mobile.nytimes.com/2015/12/04/business/dealbook/how-...