A promise of employment can be made unintentionally if an applicant gives up a lot to join your business. Suppose an individual has been working hundreds of miles away at a good job for 15 years. You hire the individual, inducing him to take his children out of their normal school and move across country. The courts may very well state that you misrepresented your intentions to this person. You may be liable for damages, even if no contract was involved.
Nobody likes to rock the boat.
That goes double for supervisors when annual performance review time rolls around. Maybe they want to be liked. Or maybe they feel that a written negative review will further demoralize an employee with attitude problems. Whatever the reason, your business ends up with documented reviews that do not adequately describe performance problems. That spells trouble after an employee is fired.