They do, when they put in 51% of the capital needed to grow the business (i.e., they bootstrapped the company). In each funding round you can generally expect 15-25% dilution, and then employees will receive options as well. By series B-C most founding teams will own less than 50% of the business.

Also, remember that different shares come with different voting rights. You can not own 51% of the economic value of a company but still have 51% of voting shares.