It's very hard to sue a corporation because the owners of said corporation are allowed to not have their personal funds tied to the business. If the assets of the sole proprietorship or partnership cannot satisfy the debt, creditors can go after each owner's personal bank account, house, etc. to make up the difference. On the other hand, if a corporation runs out of funds, its owners are usually not liable. Meaning, the owner has minimized risk in the actual business itself, anymore, other than the business itself. Trying to sue someone who can drag out court processes for years because they, personally, are not affect, is very hard to do. Now, with today's internet and such, many just settled, but that is a fairly recent development.
Earnings from a sole proprietorship are subject to self-employment taxes, which are currently a combined 13.3% on the first $106,800 of income. With a corporation, only salaries (and not profits) are subject to such taxes. This can save you thousands of dollars per year. This is a HUGE tax break, designed to help big businesses get bigger. It's just one way they have a an unfair competitive advantage over all other businesses.
A corporation has many avenues to raise capital. It can sell shares of stock and create new types of stock, such as preferred stock, with different voting or profit characteristics. Plus, investors can rest assured knowing they are not personally liable for corporate debts. It is this that helped bring about the economic crisis we are currently in. A separation from personal risk and potential profit.
Becoming incorporated is something any business can do once they reach a certain size, but it's not cheap.