"At-will" means that either both the employer and employee can break it off at any time. It's the maximum freedom for everyone involved. No company is compelled to hire anyone, why should they be compelled to continue employing someone indefinitely?
Corporate personhood is a funny artifact of English Common Law, and its present state in the US is almost unique. Much of the world views corporations rather differently than our (I'm assuming you're American) legal system does.
When you start from the premise that a corporation is an artificial construct that exists at the whim of, for the purposes intended by, and for the general good of, society as expressed through laws, including a corporation in words like "everyone", and speaking of its "freedom", becomes nonsensical.