Bethesda do mods, in a way that doesn't require modifying the game's code. They use a script language with severe limitations running in a sandbox environment. The script language can't be used to break out of it. Also, Sony doesn't allow usermade models/textures (they fear it can be used to inject and run unauthorised code). MS do allow that, but they probably have better security measures in place.Well like how Bethesda did mods if they built it in then it would be required which is why console won't use tmodloader unless required.
Also making mods like Calamity for a version of Terraria that has similar mod support as Bethesda games have, would be impossible.