Because the majority of Church Fathers talk about the rise of the 3rd and Final Holy Roman Empire (i.e. Great Monarch/Holy Pope) BEFORE the coming of the antichrist. They say that the whole world will be Catholic (which has never happened before in history), and then, after the death of such holy leaders, the world is split into 10 kingdoms (which corresponds to the Apocalypse), and the antichrist arises at a young age, who wins battles between these 10 kings, and seduces 7 of them to his side.
There is no talk of a "holy monarch" in the days AFTER antichrist, or during. Because the antichrist takes all the power.
So, logically, and as many prophecies (and Church Fathers explain) such a period of renewal, where a Holy Monarch/Holy Pope would gain power is BEFORE antichrist.
Could you provide the actual source material, so that I can understand where you get your ideas. I will now try to explain
what I think will happen.
Here is what St. Thomas, following the Church Fathers, says about the final "Roman Empire" before the coming of Antichrist:
34. First comes the revolt, which is explained in many different ways in the Gloss. And first it is explained as a revolt from faith, because later the faith would be accepted by the whole world. And this Gospel of the kingdom will be preached in the whole world (Matt 24:14). Therefore this comes before what according to Augustine has not yet been fulfilled, and afterwards many will depart from the faith, etc. In the last times certain men will depart from the faith (1 Tim 4:1). The love of many will grow cold (Matt 24:12).
Or a revolt from the Roman empire, to which the whole world was subject. And Augustine says that this is represented in Daniel by the statues (Dan 2:31), where four kingdoms are named after whom comes Christ, and that this was a fitting sign because the Roman empire was strengthened for this very purpose, namely that under its power the faith should be preached throughout the whole world.
35. But how can this be, since the nations have already withdrawn themselves from the Roman empire and yet the Antichrist has still not come?
The answer is that it is not over yet, but has changed from a temporal revolt into a spiritual revolt, as Pope Leo says in a sermon about the apostles. And so one should say that the revolt from the Roman empire should be understood not only as a revolt from the temporal but from the spiritual empire, namely from the catholic faith of the Roman church. And it is fitting that, as Christ came when the Roman empire held sway over all, so conversely a sign of the Antichrist is revolt against it.
Below is my interpretation. You are welcome to disagree.
This "final Roman Empire" is a spiritual empire, i.e. the Roman Catholic Church, as Pope Leo says above. The revolt will be the revolt against the true Catholic doctrine. The seeds of that revolt started to grow after VII and will come to full fruition with the Synod on Synodality.
The Holy Pope and Great Monarch events (which include "the Warning" and the short period of peace) will happen in the midst of the 3.5 year period, probably after the Antichrist definitively reveals himself as such.
Prior to this definitive revelation, the person who will later be understood to be the Antichrist is an Antipope (the False Prophet) laying the groundwork for the final revolt. So Antipope/False Prophet morphs into the Antichrist. They are not two different people. It is the same person before and after the final "revolt." The model for the Antichrist is Judas Iscariot. One day he is an Apostle (a false apostle). The next day he is selling out Jesus (Antichrist).
Those who respond properly to the Warning will ultimately be saved. Those who choose to, instead, follow Antichrist, because of the "operation of error," will become little copy-cat "antichrists" and will lose their souls.
The ten horns, or kings, are simply Cardinals/Bishops of the Counterfeit Catholic Church. They are "in the Beast," according to Apoc. 17:16, and "these shall hate the harlot, and shall make her desolate and naked, and shall eat her flesh, and shall burn her with fire." These bishops are the people who will carry out the edicts of the Synod on Synodality and lead the Harlot (false Catholics) into the Abyss.