Reasons I can think of that safety sausages etc may come with plastic suicide clips are a) plastic is cheaper than metal b) metal bolt snaps are more expensive to manufacture as said and have a moving part, c) safety sausages are lightweight and manufacturers think they don't need a stronger metal gate clip, and d) manufacturers of non-technical dive gear may not be all that aware. It needs a clip? Oh, let's use these cheap ones.
Sure, when I started diving and got my first safety sausage, I used the plastic suicide clip that came with the sausage.
I can remember getting the plastic gate part pulled outside the hook so that it was useless as a secure method of fastening at one point, and that was irritating to not trust the attachment point.
Later when I had reviewed why they were not the best choice, I removed them from existing gear and then bought only bolt snaps or gear with bolt snaps. YMMV.