>>7593974 (OP)Because collage is considered an art form of its own, whereas simply tracing is seen as lazy, and hiding how the sausage was actually made.
That said, tracing won't even be knocked against in the right context - As
>>7593981 points out, comic artists (not just manga) artists have been tracing for yonks, but it's almost always just props and backgrounds, characters themselves are not seen as appropriate to trace (as far as I know).
Notice how no one complains about the traced/photobashed backgrounds, yet we hear people constantly bring up the copy/pasted characters from Marvel and DC, such as this infamous page (picrel), likely because we don't really care too much about the backgrounds, and they could almost just be white voids for all we care, but the characters are the actors on the stage, and we want some effort and skill displayed by them (which means we want that from the artist).