Very few new employees try their best and give their best right away. In fact, according to an article published in Training Industry Quarterly, it takes at least 1 or 2 years before an employee is “fully productive”. This means that even after an employee has been hired, they may not be as productive as the previous employee for 2 years. You'll lose potential value for months or years after your previous employee has left.
Training for new employees is under pressure. The faster new employees can get to work, the better. In an ideal world, you should try to stay at every job for a minimum of two years. Employers need time and money to find the right candidate, especially when considering the investment they make in training and onboarding them.