Just because it can be terminated at will doesn't mean people wake up every morning going to work thinking "hmm... wonder if I'll be let go today, or hey maybe I'll quit!"

Similarly most non-marriage romantic relationship can be trivially ended "at will", but people get plenty upset when those end early than one party had anticipated.

More important though is the inherent asymmetry: it hurts employees much more to be terminated from an employer than the employer hurts when employees leave. You know, unless those employees organize and all leave at the same time. And we know well that companies put a fair bit of energy keeping that from happening.

Historically the way to resolve this asymmetry was an implicit understanding that you won't arbitrarily get terminated unless the employer has no other choice. This implicit assumption has been weakening over time.