You're assuming perfectly priced traffic, in an arbitrage-free environment where the buyers of traffic pay perfectly for the traffic, an exact amount proportional to their revenue that they will get from that traffic.

It doesn't always work that way in the real world. Some random company buying the ads at 10 cents CPM is not always getting traffic that is worth that much 100% of the time. By actually getting the entire revenue stream, the "child" company, and thus its parent would capture the true value of the traffic over a long period of time.