I am not Catholic, but it is basic Christian history as passed down from our fathers that there is no new revelation after the apostles, and no new prophets.
St. Matthew tells us in his gospel the following:
“Beware of false prophets, who come to you in sheep’s clothing, but inwardly they are ravenous wolves. 16 You will know them by their fruits. Do men gather grapes from thornbushes or figs from thistles? 17 Even so, every good tree bears good fruit, but a bad tree bears bad fruit. 18 A good tree cannot bear bad fruit, nor can a bad tree bear good fruit. 19 Every tree that does not bear good fruit is cut down and thrown into the fire. 20 Therefore by their fruits you will know them."
We could apply this to Islam or to any religion that starts after Christianity.
Islam’s theology is against the fundamental Christian theology, too: That God is in one in divinity in three persons (the Holy Trinity); that the second person of the Holy Trinity, our Lord and savior Jesus Christ was incarnate of the Holy Spirit and of the Virgin Mary (Theotokos), who truly gave birth to God the logos; that Jesus Christ was before all ages as is forever the only begotten Son of God (in the Islamic scripture, they say that God does not begat nor is he begotten; we agree with the second part only…clearly Jesus is the only-begotten, in our theology); that Jesus Christ was crucified, died, was buried, and resurrected on the third day.
These are all things that we believe to our bones and with all of our hearts and minds, and these are all things that Islam disbelieves. What possible thing can we do other than reject it? We cannot accept it. No Christian can accept Islam or Mohammed and remain a Christian. Mohammed is not a prophet for us and his religion cannot be true, for us or for anyone else.
Traditional Christianity like in the Orthodox church and the Catholic church does not have this idea like you’ve said, where something can be true for others but not actually true. Christianity says that it is true alone, and if other religions also have true things in them, they are still false because they have other things in them that disagree with Christianity. We cannot pick things out just to be nice, so we have to say “false religion” about others religions, if people ask like you did why we don’t accept something (so not just to judge and think we are superior when we are not; Christianity is true because Jesus Christ is true and God has revealed everything to us in perfection in the pure Christian faith). Actually I think this is very similar to how Islam thinks about itself. If the Jews don’t believe in that, then they would be the “odd” religion among the three of us.