Anything that creates obligations essentially has to be done as a company. Open-source programmers typically tend to be leery of taking on projects where they have to do something, with good reason. Also, contracts generally aren't enforceable if there is no money (or other "consideration") changing hands. Anything where you want to be sure that something gets done in the future basically needs to go through the real economy.

The examples cited so far are basically special cases of this. Google needs to be able to enter into contracts for hosting, bandwidth, power, uptime, etc, so it needs to be a real corporation. When you entrust sensitive personal data, you want an ongoing obligation to keep that data secure, which you can't get with open-source. Additional examples might be corporate support contracts, support for certain platforms, and any service dealing with payment or finances.

Basically, open-source is good for sharing what people have done, while corporations are good for determining what people will do.