[fixermark]

It's probably a lot less about anti-trust and a lot more about bankruptcy protection.

With the companies logically divided, Alphabet can choose devote $XYZ money to some specific initiative and have a guarantee that they're liable for AT MOST that much money. If the sub-company completely mis-manages its assets and goes belly-up, its failure doesn't impact e.g. Google or Waymo (in any deeper way than opportunity cost of the money going to the other sub-company instead of those). Bankruptcy responsibility stops at the owning company and doesn't trickle up to the assets of the company that owns that company, in general.

It's a common pattern for movie studios---Hollywood studios build out a short-lived company to own every film production to insulate themselves from liability and financial disaster.