Is that a serious question? The employee/employer relationship is not a 1:1 equal footing super best friendship.
Yes, it's a serious question. Apparently you are too stupid to realize it.
And employee/employer relationship is a contract... In contract law, both sides have to in good faith complete their end of the bargain, otherwise they are considered in breach.
If you had some siders working on your house, and complete half the job. Are you as their employer required to pay them for services? Do they need to refund your money if they did quit?
It's an employee/employer relationship. But in the above case, it's not at-will and how will play out is based upon the contract that is signed by both parties.
The point of at-will is to prevent wishy-washy agreements over long term employment where it's impossible to create a contract as there is always a moving target. But the point is: Employer can fire your ass when they want, and you can quit at any time without a breach of contract.
But with your warped sense of reality, only one side is required to be in breach of contract - and is liable for severance. Thankfully, your stupidity isn't part of law.