This clearly is an understatement about the effects of the law, first as to Jews, and then enlarged to all mankind. This is clearly established by the statement in verse 13:
For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified.
So, what hearer of the law under the law was ever justified by the law, other than the giver of the law, Jesus, the Christ?
Then, what about those not under the law? Remember, God said in Exodus 20:1-26 KJV:
And God spake all these words, saying, I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage. Thou shalt... (on and on). Ye shall not make with me gods of silver, neither shall ye make unto you gods of gold. An altar of earth thou shalt make unto me, and shalt sacrifice thereon thy burnt offerings, and thy peace offerings, thy sheep, and thine oxen: in all places where I record my name I will come unto thee, and I will bless thee. And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it. Neither shalt thou go up by steps unto mine altar, that thy nakedness be not discovered thereon.
These were bullet points of the law that required chapter after chapter of our now-separated-by-chapter text to set out strict specifics as to that law. These specifics cause headaches of Jews who were schooled in that law to attempt to understand so as to try vainly to keep that law.
CONTINUING, in Romans 3:1-9 Paul compares Jews with Gentiles as to advantage:
What advantage then hath the Jew? or what profit is there of circumcision?
and ANSWERING--in perfect context--in the very next verse:
Much every way: chiefly, because that unto them were committed the oracles of God.
The Jews were those hearers of the law referred to in the OP's question.
What about the Gentiles? The OP's text cites verse 14:
For when Gentiles, who do not have the law, by nature do what the law requires, ... .
Now that's something way beyond the ability of any man on earth to do. Remember, The verse before just said, "Not the hearers of the law"
When did God ever bring a Gentile out of the land of Egypt? When did any Gentile ever do those things that are contained in the entire law, let alone only the bullet points of the ten commandments, ESPECIALLY without ever hearing those things that the "law saith".
Paul concludes with this answer to the OP's question, in Romans 3:19-20 KJV:
Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
(That is "all the world", not just the hearers--the Jews--shut your mouth, you are guilty)
How could any man, having a sinful nature, possibly do, by his sinful nature, those righteous things contained in the law?
THEREFORE by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.