It is not quite as easy as you suppose. The Bank Holding Company act of 1956 (as amended in 1970) places any company that owns 25% or more of a bank under the regulatory supervision of the Fed. The holding company rules for Broker/Dealers (investment banks and retail/institutional brokerages) are less airtight but not insignificant.

I worked on Wall Street for years, many of the them on the business side. Fed supervision is no joke.