Amaterasu shows up one night, head tilted. Sunleash glows gently on her back, but is clearly perfectly fine. No, she's here because she's pretty sure she can help out, a bit. The Celestial Brush can't be used constantly, and she probably couldn't keep up with everything that needs repairs... And there are several forms of damage that it is unsuited for, even.

But still... "I'd like to try and help, if I can? Among my techniques is Rejuvenation, which makes things whole again." There are so many reasons that she can't do this all the time... But it's important to use the brush as it is meant to be used. Not just as a weapon.