[Q] Why can't alter the su binary to have system access only instead of root

Hi,

Imagine a scenario where you have just system access on android , and you want to continually have it, so you use the su binary, with the correct permission and the setuid flag on, and the owner being system, you do:
you have the ability to change the owner to system as you do when rooting, and you also set the appropriate permission as you do when rooting,

06775 being the permission code to all have execution permission, and the setuid flag on.

But when dealing with system permission that doesn't work. it only works with root.

I know it's a weird question, but if any could answer it, id be glad .... thanks