Actually, a few businessmen in my home town got together and started a bank. Tesla Motors is an example of a car company started, according to Wikipedia, by a couple of guys with personal funds (while they created the business plan). Regularly, although rarely, companies start out small, attack some market, and then grow into the multinational corporations.
some markets are more closed than ever.
I definitely don't agree with this, at least not the examples that you mentioned. Technology makes it easier in all industries. Biotechs in pharma. Five Guys Burger and Fries in restaurants. Boutique hedge funds in finance. While I don't know the oil industry, it sounds like there are a lot of areas where a small startup could focus on and eventually become a bigger company, like owning a drilling rig (and run it well and according to best practices), or underwater surveying, or something.