I wonder why some people need a rebirth to be strong or to take revenge, there are things that you do when cornered, not just to stay there and be bullied to death
For a lot of people. Especially "good people". Events hit them sideways like a truck, and they couldn't see it coming at the moment. But looking back, what led to that event is perfectly clear (Hindsight is always 20/20). And acting when corned, especially if dealing with people who're more powerful, more connected, and / or better at manipulating the opinion of others, without laying groundwork. Is actually pretty difficult.
Heck even Pro-Revenge Subreddits, needs a bit of build up and resources.
So for people like in these stories (Where social classes exist. Or there's a massive difference in social class.) Resetting to a point in the past where they are in possition to set up the foundation for revenge, and with the gift of hindsight. Makes revenge possible.
As for being strong. For lots of them, death is the wake up call they need. Since there are quite a lot of people who're made to feel weak and powerless until death is staring them in the eyes.