They are not able to contract with anyone. I recently read a news story about a producer winning a multimillion-dollar judgment against an actress based on her violation of an oral contract. The problem with oral contracts is that they are very hard to prove. What you have in your mind about what was promised may not be as certain as you would like it to be, and later on may be shaded by what actually happened when it was time to carry out the contract. Things may have come up that you never anticipated when you first made the agreement.
People make oral contracts all the time.
Ours used to be a handshake society. Complex business deals were finalized on the basis of a handshake, and details were worked out later. Many people think commerce still runs this way. Not too long ago, a single mother of two appeared in my courtroom claiming that she had been taken advantage of by a home remodeling contractor. The deal made on a handshake has not been a wise course to take since about 1860, when legal cases first started to be reported in this country.