I can't say for certain, but I can make some inferences.

A saint is someone who has been sanctified. That is, they have been made holy or sacred, or have been purified. This process implies beginning from a state this is something less.

Angels are beings that exist in Heaven, and have always existed in heaven. This implies that they are without sin, and have been from the time of their creation. It would be impossible to sanctify such a being, because there would be nothing to change. They are already pure. The good news is that this line of reasoning also implies that angels already have sainthood in the ways that matter most.

This does bring up the case of fallen Angels. I suppose it may be possible for a fallen angel to repent, and then be redeemed and sanctified. It may even have happened... but there is no record of such an event in scripture.

Simplest answer is probably grammatical: One Greek word hagios becomes two in English, "holy" (adjective) and "saint" (noun, title). So what could have been translated "Holy Michael the Archangel" was translated "Saint Michael the Archangel." Of course it also came through Latin where you've got the word sanctus.

By canonizing some of the faithful, i.e., by solemnly proclaiming that they practiced heroic virtue and lived in fidelity to God's grace, the Church recognizes the power of the Spirit of holiness within her and sustains the hope of believers by proposing the saints to them as models and intercessors.

CCC 828

Some angels have been recognized as saints because of their heroic virtue and fidelity to God's grace, and these angels can also fulfill the role of model and intercessor. One particular example is Saint Michael the Archangel, whom we know about from the book of Revelation.

Not all angels are saints because not all angels have heroic virtue and fidelity to God's grace. (Fallen angels are an example of this.) Also, while many angels are in heaven, there are few angels who are saints because we do not know enough about them to canonize them. Saint Michael is one of the few angels that we do know a lot about.

Also note that while we believe all saints are in heaven, simply being in heaven does not mean that a person is recognized as a saint.

Whether Angels can be considered Saints

Objection 1. It would seem that angels cannot be numbered among the Saints because during the Holy Sacrifice of the Mass, the one presiding says:

...all the Angels and Saints

and furthermore, at benediction the one presiding says (and the people respond):

blessed be God in His Angels and in His Saints

So these would seem to be two separate groups of people

Objection 2. Further, the Holy Church has shown throughout Salvation History that she only canonizes those who have died in the Grace of God. Angels have no capacity for death and therefore cannot be canonized as such.

Objection 3. Further, Holy Mother Church makes a distinction between those whom she knows, through:

1. objective means (posthumous miracles)
2. through her own power of binding and loosing (as is the case with St. Pope John XXIII)
3. by right of dying specifically for Christ (martyrdom). In this case one does not have the ability to recant the grace freely given. This makes their death the sacrifice "beyond which there is no greater love".

On the Contrary the Catholic Church, which apparently is the arbiter of who is numbered among the Saints, has consistently referred to the angels as Saints. Mor Gabriel Monestary is one of the oldest surviving Churches in the world, dedicated to St. Gabriel.

I answer that, the term saint is a qualifier used to mean holy but a canonized Saint is an office, as is Angel. Angel is a job title meaning messenger of God. Just as a person can be both a computer programmer and a father, one can be an Angel and a Saint.

Reply to Objection 1 the words used in the sacred liturgies, whereas they are unsurpassed in beauty and reverence, are not, in and of themselves, theological treatises. Therefore, if you hear Jesus referred to as thrice Holy, that isn't to say He isn't also holy four times over as well.

Reply to Objection 2 It is true that the relatively recently established norms of canonization are more or less ignored when it comes to Biblical figures, when it comes to those who do not need the grace of Christ's redemption to save them from death, the Ox says

Now a multitude ordained to one end, with distinct acts and duties, may be metaphorically called one body. But it is manifest that both men and angels are ordained to one end, which is the glory of the Divine fruition.

Summa III:62:4

and so he lumps us all together in Heaven where Christ is the Head and we all (the Church) are His Body. And so, a thing that separates us from the Angels (the ability to die) does not separate us from becoming Saints any more than it separates Angels from being Saints.

Reply to Objection 3. Lastly, the Church, through Holy Scripture knows which of the Angels clung to the true light (The Trinity) or the false light (Lucifer) because those who clung to the truth are the heroes of the Bible. St. Raphael aids Tobias on his journey and saves him, his future wife's honor and his father. St. Michael wrested from Lucifer the body of Moses and preserved the holy man from scorn. St. Gabriel, best of all, announced the birth of Christ to Mary. But, to even accomplish this task, they must have gotten their orders from God Himself and therefore "merited beatitude" as the Ox says:

...ultimate beatitude exceeds both the angelic and the human nature. It remains, then, that both man and angel merited their beatitude.

Summa III:8:4