Companies that recruit poorly tend to train poorly as well. Training is not a one shot enterprise, but an on-going investment to reinforce and advance skills and attitudes. Employers often get caught up in a vicious cycle. They recruit poorly, then fail to allocate sufficient training resources. Employees' performance suffers, their satisfaction level is low, and they leave giving the employer the opportunity to start the cycle again.
Employers ought to treat employees as an investment rather than an expense.
Reasonable people will accomplish a reasonable amount in a reasonable time period. But, you cannot put ten quarts of water into a five-quart container. There is nothing wrong with shifting work from former employees to those who remain. Our capacity to produce will sometimes be enhanced as we take on more responsibilities. But there is a limit.