But, that was the whole idea behind MGR. I think most MGS fans missed that point, because they were expecting a "serious" story, for a game that never intended to have one. Funny thing is, none of the MGS games have ever delivered a serious plot (despite Kojima's dismal attempts to include one), and they're all filled with nonsensical rubbish.
As for Raiden, he already has a backstory that was thoroughly explained in MGS 2 and MGS 4. "Rising" touched upon his upbringing as a child soldier, and it was more about him reconciling with his Jack the Ripper nature. Psychologically, he becomes a "whole" person at the end of the game since he finally accepts himself for who he is. He's essentially taken the mantle from Solid Snake in MGS 2 - becoming the wise soldier that forges his own path forward.