We have quite a lot of employee protections (not as much as Europe but a lot more than people realize), it's the enforcement that is the issue. While you can be fired without notice for any legal reason, if you are fired for an illegal reason or an illegal reason played a role in their decision to fire you, you can get quite a nice settlement from that. However, if you are fired without a good reason, the employer has to pay for your unemployment, so the majority of employers will only fire an employee if it falls under a reason that makes you ineligible for unemployment like poor performance or attendance (and labor attorneys can often sniff out when an employer is lying about it to screw you out of unemployment). Contrary to several other countries, employees can just quit without notice or even informing their employer, as at will employment goes both ways
Edit: Lots of other replies mention this, so I'm just repeating things...
"At-will" employment is actually a state-level law, and at least one state (Montana) isn't an at-will state. That doesn't invalidate your general points, just think it's an interesting tidbit.