We tend to think everyone competing dislikes each other, but in reality many fields have a small pool of experienced works, so they only have a few companies to choose between. Combine that with the modern HR strategy of prioritizing promoting from outside the company, and add on California not recognizing non-compete agreements, and you end up with a pool of workers that have mostly been co-workers with each other, at some point in the past. The higher up you get, the smaller the pool, so you end up with a lot of overlap.
My favorite nearby shopping center has a Trader Joe's and a Grocer Outlet, with the former being founded by Joe Coulombe and the latter growing significantly with him on their board. (See his book: Becoming Trader Joe) He had a lot of influence on both, with both still having a noticeably similar obsession with cheap wine and a short time of availability for any given item, but they're both different enough to warrant being in the same shopping center, and are both worth visiting.
Paper: https://columbialawreview.org/content/anticompetitive-direct...