Lol... If you believe that then demand an employment agreement that states that. Almost all of society functions through agreements. If your employment agreement states employment is at will then you better believe that they mean it.
Unless someone is quite high up in the company or in some type of special position (in house consul for example) almost no one will have an employment agreement.
I've had an employment agreement for literally every job I've held in my career, from the very first when I was still wet behind the ears.
You have to sign agreements to work at pretty much anywhere in the US... From Wendys all the way to the White House.