The tragedy/hardship gives the MC reason. Normal people with basic, boring, easy lives typically don't try to unsettle themselves from it. MC's with baggage are the opposite - they seek better, know what it means to be at zero, and they are more willing to give everything to ensure they never go back.
Not saying you can't have an MC that's all sunshine and rainbows from start to finish. There are plenty of stories like that. But an MC who always has it is, always has the answer given to them, and never truly struggles isn't someone I would want as the face of my story. They're someone who hasn't learned the hard way and is likely either too blind or inexperienced with life's true nature.
If i may, a tragic backstory isn't needed. i mean, sure if your MC is already 50, then maybe they should've already suffered some. but, if we take a look at my current running story, Aise, the MC is 15. he has lived a fairly decent life so far, raised in a two parent home, good friends, and overall a happy life. no tragic backstory. but then he gets forced into adventure. an outside force dictates that he adventure.
additionally, the struggle he faces isn't solely an external struggle. sure, that is the catalyst, but the growth i'm going for is internal. it's man-vs-self. the readers get to watch him grow and learn, rather than already having learned, which is what you seem to suggest.
sure, an older MC or an MC with a difficult life should face more external struggles, as they don't much much internal growth left to do. but it is possible to have a compelling story where the primary focus is internal struggle and growth.
