It may come as a relief to dismiss an employee who wasn't performing, was disruptive, or had a bad attitude, but it hurts to lose a star performer who chooses to go.
After the time and investment in finding the right person for the role and developing their on-the-job skills, it's back to drafting the job vacancy ad, while their expertise and institutional knowledge heads elsewhere.
It may not reflect badly on the company if the person's reason for leaving was personal, such as to raise children or move to another part of the country, or they decided to change careers or further their education.
It does very little for the business' long-term prospects, however, if it is losing people because of factors it can control, such as the relationship between workers and management, the company culture, or staff feeling overworked and unappreciated.
Talented people want to apply their skills, and expect to be recognised and rewarded for it. They want opportunities to develop their expertise and experience by doing challenging, meaningful work that helps the company achieve its goals.
If they are given reason to feel the company doesn't care about them or their career, they will have little reason to care about the company. If employees become unmotivated and disengaged, who could blame them for finding another position somewhere else?