Suggestion Superior Transmutation Stones
The different armor weights have different meshes. To change one to another means each piece has to be individually redesigned for the other armor weights. There are problems with where seams are in the different armor weights and how many dye channels there are which make the different weights incompatible without this redesign.
Source (page 18).
……..there are some fundamental incompatible things between weight classes. (part of how we set up every armor to allow many dye channels and styles per piece).. There really is no way at this point over six years since we started development to make absolutely everything work together.
ANet may give it to you.
As J.a.f.w. says, it’s a technical difference preventing people from mixing armor weights, not just a “classes should look like their weight” restriction.
Outfits let light classes look heavy and heavy classes look light, so it really does seem to be a technical problem, rather than a policy one.