Isn't it the most heart-rendering thing when a child is finally old enough that there is nothing a parent can do to stop them leaving to go to war?
That feeling of not even knowing the fate of your own child, who you raised and cared for and know intimately, perhaps better than anyone. That breeds a kind of bitterness that without a target for hatred, can truly destroy a person
If you can get that down in writing, what a way to start a story!
It is understandable though that starting on such a dark and sombre note is not going to let you do the whole "seems like a grand adventure at first, but as things turn darker, MC grows correspondingly to face those challenges. I.e. coming of age"
Equally an orphan instantly makes the MC into an underdog character, gives them something we can instantly connect with and is universal (love for parents), makes it easier to build a sense of humbleness (humility?) that again is easy to connect with, and makes their struggle seem all the more corageous. Plus you can throw in 'issues' that everyone will have to deal with at some point, and are less likely to make the reader uncomfortable versus tackling heavier subjects such as racism or sexual exploitation head on.
Finally it is abstract. The death is vague and in the past. There is no need to look the MC's mother in the eyes one last time before they turn and get on the bus/train/spaceship that whisks them away to boot camp