Wither skeleton bones

Something I don't understand is why every time I kill a wither skeleton they give me normal skeleton bones.

That's why my request is that instead of normal skeleton bones, they give wither skeleton bones, that is, but black. It wouldn't cost that much because it's just a matter of changing the texture and giving it to the wither skeletons.

Also, with these eggs we can make wither skeleton bone powder and with this we could accelerate the growth of nether plants such as warts, trees, etc. As white egg powder accelerates the growth of overworld plants, the black ones do the same but in the nether.

I would like them to add that idea to Minecraft. I hope they do it pls.