Why do (Counter) Guardians only move after the event has taken place?

The following is quoted from Rin in the UBW route.

"That's because, the time at which the Heroic Spirit is summoned, that place has already been turned into a field of death. The place the Heroic Spirit, the Guardian appears, is nothing other than hell."

As can be understood from this passage, the conditions under which a Guardian appear are, in most conditions, have reached the point of no return, and in truth, their job is to finish off that area to prevent the damage from expanding.
The dispatch of Counter Guardians probably implies that, in that situation, just by being a counter, that much power is needed, thus the occurred incident is limited to those really dangerous things.

There was one doubt I had regarding this. This was different from the impression of the explanation of the Counter Force of Humanity in Kara no Kyoukai.
According to this, the ability to detect threats is extremely high. In fact, it seemed as if it moved to stop things from turning into a major disaster as soon as possible. On the other hand, the Guardians in Fate are only sent afterwards, and are different in this aspect.

Regarding this, there is the explanation by Kotomine on Anti-Heroes and something that Archer said at the first appearance of the "shadow" in the Sakura route that might produce a somewhat sensible interpretation.

".......While being something born by humanity, something that develops without the meddling of humans.
This contradiction itself is an "enemy of the world", exempted from the pressure of all counter forces."

The first thing to address is the explanation of Anti-Heroes by Kotomine. He says, "exempted from the pressure of Counter Forces". If we interpret it as is, this should mean that the Counter Forces won't be activated in regards to it (at least, while still in the maturing stages).

"....Hm. Even when I am summoned as a Servant, I'll still have to be fighting against that, huh."
(cut)
"Ah. You still haven't become a Guardian yet. Then, you wouldn't have had the opportunity to face the likes of that.
.....Dammit. To think, that no matter where I am, my job ends up being the same."

The next is what Archer said. Going from this, the enemies that Counter Guardians had fought mostly are the likes of things like Angra Mainyu.
And not just as simple as that, but judging from what he said to Saber, it can be interpreted that other than Guardians, there are virtually no beings that have confronted those sort of threats.
That might go without saying. If they are the sort of entities as stated within the game, there are no survivors once the Guardian is summoned. Then, that would mean that there are no witnesses left that had seen the enemy they were fighting.

Also, putting what both Kotomine and Archer said together, since Anti-Heroes are things born from humans, ultimately, the Counter Force of Humanity will be mobilized. However, as these are things that develop without the interference of humans, while they're developing, they can't be monitored. That's why, it's only after a great deal of damage has been generated for the first time that the anomaly can be detected.
Further, to respond to this, the normal method won't make it. Therefore, in this kind of situation, Guardians are sent out for the most part. This probably was what was meant.

In conclusion, it could be said that, because Guardians are especially active as counters, in comparison to the things called Counter Forces, to "those threats that are difficult to detect before-hand and are oflarge scale, like the activity of anti-hero like entities", they are ultimately beings that are for the most part sent at the point when it's too late.

I felt that it was weird how the Counter Force in Fate and one in Rakkyo were different, but the one in Fate had only talked about the Counter Guardians, and I had mistaken it for being a general comment of all things related to the Counter Forces.