There are different types of weather in your life, from autumn to winter and spring to summer. Most stories will go from winter to summer because what would a story be without going from hardship to happiness?
Unfortunately, some stories do not end with happiness or summer but sadness and winter. So to change the ending of a sad story, authors will send in their special agents, transmigrators, everyone loves them.
They go and change everything for the better. They are always better than even the original owners of the body they possessed who were always useless, stupid, dumb, ugly, etc. (all the bad qualities a human being could have) before their transmigrates arrived.
Have you dear readers ever thought of how the true owners of those bodies feel when they look at the actions of those possessors? Or how the family would react if they knew their person was some unknown person. The possessive husband and kids, the siscon brother, or the loving mother. Will they treat those transmigrators the same way, worse than before?