Is that all you can come up with?
Paul presents at least 8 different categories, in his letter to the Romans, alone.
The Law of Faith (Ch 3)
A Different Law (Ch 7)
The Law of My Mind (Ch 7)
The Law of Sin (Sin's Law) (Ch 7)
God's Law (Ch 7)
The Spirit's Law of Life (Ch 8)
The Law of Sin and Death (Ch 8)
The Law of Righteousness (Ch 9)
I gather then the "why" of the OP is merely rhetorical...
I could come up with a lot more than that, but not sure if it's going to be fruitful yet tbh. Jury is out on that.
There's no reason to assume these eight "categories" are categories at all in Paul's mind. Why would a "different law" in Rom 7 be a "category" of Law? You are looking for technicalities where there is none. The argument in Romans 7 is not to create four different categories but to give examples of how the Law (one Law) works.
Paul in Romans 8 shows that a Law of Life, "the Law of the Spirit of Life" is bigger than the Law - the Law that brings sin and condemns to death.
The Law of Righteousness in chapter 9 is a discussion on how Israel pursued the Law as a way to righteousness.
Romans 3 is the most exciting of all, showing how the Law of Faith now swallows up the old Law of condemnation.
The reason why the Law remains in effect for ever is because the Law "always accuses" and God's morality has not changed. But the
use of the Law was never to reach righteousness but to expose unrighteousness, whereas faith in Jesus leads to righteousness. That is Paul's argument. All these categories are ways of explaining the same thing and are not "categories" at all.
Yahshua was obedient to his father's instruction. He told us to follow him in his example.
Jesus is a Messiah to be believed and trusted in, not an example to emulate. He is the fulfilment of Israel's story and mission, undoing what Adam did and recapitulating Israel's story - this time into success. No one can emulate Him, his mission was unique and once-for-all and His job done. Now, we walk in His Spirit and that leads us to righteousness.
I'm happy with my gospel of freedom and righteousness in Christ Jesus, the one who died for me and was raised for me. He was born under the law (Gal 4:4) and then died, being set free from it (as Romans 7 argues); in Him I live and through baptism have died to the Law and to Sin and to Death and to the Devil and have been raised with Him into new life, free from the Law but now in the Spirit. That's good news and I'll take that thanks very much and not go back to trying to attain a righteousness under the law, for then Christ died for nothing.