Actually there are lots of factors impacting horizontal and vertical integration in the capitalist economy. In general you are right that "external" restraints

Actually there are lots of factors impacting horizontal and vertical integration in the capitalist economy. In general you are right that "external" restraints are needed to prevent a firm from competing so aggressively or moving to integrate horizontally. However, the external restraints are far and away most often free market forces. Economies of scale are nothing to laugh at, especially in ventures with very high fixed costs. However, integration is also high risk because it comes with significant downsides. I don't care to analyze the US cellular market, but I would be curious to what extent the way the government has regulated this market has pushed it toward mergers. And perhaps that is a good thing to have a small number of national cell carriers. People who grew up in the early telephone era would probably agree its better than having to navigate the byzantine maze of regional networks and long distance charges. There are other mostly unregulated wireless markets that might serve as interesting case studies though. Wireless ISPs have been a booming business for decades and is still extremely dispersed in ownership. They're facing some disruption now from Starlink but that is likely years away from posing a serious industry challenge.