Analyzed well here: https://yalelawjournal.org/pdf/e.710.Khan.805_zuvfyyeh.pdf
They aren't a majority in any other market segment.
Too many markets are utterly dominated by one or two big players. I know it’s a tricky problem because market share is hard to define (Does Amazon have 80% share of e-commerce? Or 30% share of all retail?) but I think we would be better off if there were a more aggressive set of rules about anti-competitive behavior that automatically applied to these huge firms, which didn’t rely so much on subjective judgment.