I''m pretty sure that''s not done because of the 555 565 format thing.. even if you convert the green value from a 6 pixel to a 5 pixel format I think you loose quality whereas you don''t loose that quality between 24bit and those two version... 24bit is just nicer to work with because it''s all bytes... no 5 bits here 6 bits there, but it''s hard to tell the difference between 24bit and 16bpp in games... (Photos you can tell if you are looking for it...)
But I guess you could implement your own file system. But you''d be on your own. Besides what real advantage is it? At most you''ll never load more than like 64 mb of graphics and on a HD or CDROM that is insignificant to the space... IMHO.
See ya,
Ben