Or perhaps corporations grow large because they serve diverse needs. Much like users only need 10% of Microsoft Word, yet many cohorts often need a different 10% slice.
Or we lack anti-trust controls, so rent seekers are soaking up markets.
Likely. How big each "slice" is would be good to know. I wish someone would study it in depth, if it's even possible to fairly analyze.
I don't necessarily think your analogy with MS word is a fair or correct one, FWIW, only because once you build something the functionality is able to be distributed and replicated.
Employees require active up keep in a way that software really doesn't.
Or we lack anti-trust controls, so rent seekers are soaking up markets.
Maybe a combination of all three.