Comment by orisho

I believe it's because if you are easily replaceable, then screwing up means you're not worth the trouble. If you aren't easily replaceable (whether it's because you have demonstrated you're a good employee or you're working a high-demand role), you are worth the trouble and you'll get more chances. There are other reasons too, such as jurisdictions where suing after being laid off is more common, which makes more chances, PIP and severance packages more likely.