Really? You think its weird that someone leaving impacts that person more than their manager, team, or company?
A manager doesn't care that you leave because he still gets to bring home a salary, all they have to do is hire another person and maybe cut back on scope for a little while. When someone is layed off or fired they have to go find a new job, their income streams dry up, and are forced to rely on savings. People often have long term financial commitments they cannot back out of, and not knowing how you'll make rent, if you can afford your child's school fees, or even maybe having to cut back on how much you eat, _is stressful_ and emotionally taxing.
Are you fucking kidding me? "Which is a bit weird" I'll tell you what's weird: management who doesn't understand that their employees rely on them more than they rely on the employee.
Of course its fine when someone chooses to leave their job, they've made contingencies and planned around it. Whether it's through savings, another job, or the lottery, people have at least some idea of their plan when they leave a job.