Does any D&D system have it set in stone?
No, I guess some cases it does because low int means it lacks the capacity to even understand the basic concepts and its just instincts but animals tend to have(or had) (T)N alignment since it doesnt apply to then.
The traditional Goblin society is however evil and so most Goblins will be evil, not all but the society itself will lend to create individuals that will be of that alignment since because thats how things work among Goblins, besides Goblins are folklore creatures and so they should reflect their origins and not having FUCKING HIPSTERS think they are so cool and AGAINST THE MAN and change then because YOU CANT TELL ME WHAT TO DO DAD!
If at one point your "Goblin" is nothing but a Gnome or a Dwarf well then, congratulations on your fedora and your YT D&D channel.