Scripture explains why:
“ [...] he will banish ungodliness from Jacob” “and this will be my covenant with them when I take away their sins.” Romans 11:26b-27... still to be fulfilled when Paul wrote these lines
in future tense.
Those aren't originally Paul's words.
In Romans 11:26-27, Paul is quoting Isaiah's words (recorded in Is 59:20). These words were prophecy when Isaiah spoke them - but if you're seeing these words as still yet to be fulfilled, I believe you're completely missing what God had already accomplished by Paul's time:
Isaiah 59:20 ~ "A Redeemer will come to Zion, And to those who turn from transgression in Jacob," declares the LORD.
Romans 11:26 ~ And in this way all Israel will be saved, as it is written “The Deliverer will come from Zion; He will remove godlessness from Jacob.
.....the question a person should ask is, "in WHAT way?". That's answered in the previous verse:
Romans 11:25 ~ For I do not want you to be ignorant, brothers, of this mystery, so that you may not be wise in yourselves: A hardening in part has happened to Israel, until the fullness of the Gentiles may come in.
"ignorant of this mystery" - the definition of a mystery is given in chapter 16 verse 25-26:
Romans 16:25-26 ~ Now all glory to God, who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages but has now been disclosed and through the prophetic writings has been made known to all nations, according to the command of the eternal God, to bring about the obedience of faith.
A mystery is something that's been hidden in the past and is now revealed in the Scripture.
In Ephesians 3 Paul unfolds in detail this mystery:
Ephesians 3:1-3 ~ For this reason I, Paul, a prisoner of Christ Yeshua on behalf of you Gentiles— assuming that you have heard of the stewardship of God's grace that was given to me for you, how the mystery was made known to me by revelation, as I have written briefly.
What is the mystery?
Ephesians 3:5-6 ~ which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Yeshua through the gospel.
The mystery is that Jew and Gentile were brought together now in one body.