The most common misconception that most corporate have is the idea that everyone can be replaced in a day ! I believe, from the country that I reside in, that employees tend to be exploited with this idea. In India, we had a huge exponential rise in the people who wanted to do engineering in the past few years.
Since the number of job seekers are significantly higher than the job opportunities, most people are ready to work even for a very low wage to earn experience or thinking that earning a bit is even better than earning nothing at all. This had resulted in undervalued work.
The corporate world always wants to get the maximum value of money that they pay for a person. They, in fact, do not bother about the person as a whole. The money and organisation is placed well above the humanitarian consideration. Corporate world often (mostly the people I have met) thinks that, replacing an employee is quite easy and does not impact the work, as the new person also has the same skill-set as the one being replaced.
When viewed from the other perspective, not all information and knowledge will be shared to the organisation by the individual. He might have missed things, or may be thought the information was not useful. The employee who had experience in the project, would be thereby, knowing a lot more, maybe even more specific information regarding the project than the senior or the manager as the work done is more at the lower ends of the organisation, when it comes to technical sections. Since the new comer would have no idea regarding this, he would either miss information or even if he does catch up, will spend a few days or months catching up depending on the depth of the project. Well, time is money.
Most people think that, hiring a new employee with lower salary and then completing the project was an achievement. It might be so, or maybe it seemed so. Maybe the old employee, who had to be paid more, would have helped you complete the work much earlier, with an improved set of features. No one can be sure, but then not everyone fits in !