Spoilers: it's because our regulatory, legislative, and judicial infrastructure exists to defend monied interests. We're incapable of taking collective action that reduces private property accumulation, so no monopoly busting.
In both 2017 and 2018, Alphabet is the first listed corporation[0], which is probably the reason I see the claim made, most of the rest are associations.
[0]If you ignore BC/BS, which is a "federation of 36 independent companies", though that is largely a way to deal with the law requiring health insurance be in-state. If you want to, I'd accept saying Alphabet is the second corporation on the list, if we're being real.