Because the Bible defines both salvation (Eph 2:8) and eternal life (Rom 6:23) as gifts.
And Paul defined "work" as something that creates a debt owed (Rom 4:4). Believing God's promise does not create a debt owed. It's that simple.
4 Now to him who works, the wages are not counted as grace but as debt.
Romans 4:4
Yes agreed, the work that is described here is work in order to receive wages.
Is that the only description of the word "works" in scripture?
Works to receive wages.
Works of the law.
Good works of charity.
The work of obedience in responses to the Voice of God.
I believe we are all in agreement that we can not work as one works to receive wages, to earn eternal life.
However, Paul just got through writing these words from Romans Chapter 2.
1 Therefore you are inexcusable, O man, whoever you are who judge, for in whatever you judge another you condemn yourself; for you who judge practice the same things.
2 But we know that the judgment of God is according to truth against those who practice such things.
3 And do you think this, O man, you who judge those practicing such things, and doing the same, that you will escape the judgment of God?
4 Or do you despise the riches of His goodness, forbearance, and longsuffering, not knowing that the goodness of God leads you to repentance?
5 But in accordance with your hardness and your impenitent heart you are treasuring up for yourself wrath in the day of wrath and revelation of the righteous judgment of God,
6 who "will render to each one according to his deeds":
7 eternal life to those who by patient continuance in doing good seek for glory, honor, and immortality;
8 but to those who are self-seeking and do not obey the truth, but obey unrighteousness--indignation and wrath,
9 tribulation and anguish, on every soul of man who does evil, of the Jew first and also of the Greek;
10 but glory, honor, and peace
to everyone who works what is good, to the Jew first and also to the Greek.
Romans 2:1-10
JLB