I think depends on the culture/country.

In the US definitely the agreements are more flexible.

In Europe (and probably elsewhere) I feel that the expectation was (at least some years ago) that once you join a company the relation is stronger.

Each variation has pluses and minuses, for companies and individuals.

Humans are tribal creatures, and joining a company and working with the same people every day creates bonds, a feeling of loyalty, and feelings of betrayal and unease when told they no longer belong.