At least, I think that's the way it's supposed to work. I used to work at Goldman, and at some point, someone told me that the number of legal entities was roughly the number of employees. I'm not sure how true that was, but I'd believe it.
There are some good reasons to allow these separate legal entities, to allow orderly partial collapses of businesses to reduce the risk of cascading systemic failures. There's also something to be said about either keeping businesses small enough that failure of a handful of them won't collapse markets, or else single-purpose enough that they have less direct effects on multiple markets. The flip side is that diversification across markets should tend to stabilize firms, and larger firms are also more able to effectively diversify.
Real estate is a special situation with lots of subcontracting. They can make it work. In big multi-unit residential cities like NYC and Chicago, the doorman in that high-rise is in a union that provides all his benefits and has a set pay scale. The building ownership just contracts all that stuff out to the union. But that doorman can go work in some other building owned by some other owner too. You don't want that happening with your corporate employees developing and managing your consumer products.