Q. Does anyone know how long this Catholic practice has been in place and why it was instituted? To be honest, it seems to me that the information I have come across to support it is flimsy at best.
A. It has been practice, documented from the very earliest centuries of Christianity. This practice predates the canonization of the Bible. Not only that but everyone who has lost a loved one longs commune with them in a holy way.
Clement of Alexandria
“In this way is he [the true Christian] always pure for prayer. He also prays in the society of angels, as being already of angelic rank, and he is never out of their holy keeping; and though he pray alone, he has the choir of the saints standing with him [in prayer]” (Miscellanies 7:12 [A.D. 208]).
“But not the high priest [Christ] alone prays for those who pray sincerely, but also the angels . . . as also the souls of the saints who have already fallen asleep” (Prayer 11 [A.D. 233]).
Cyprian of Carthage
“Let us remember one another in concord and unanimity. Let us on both sides [of death] always pray for one another. Let us relieve burdens and afflictions by mutual love, that if one of us, by the swiftness of divine condescension, shall go hence first, our love may continue in the presence of the Lord, and our prayers for our brethren and sisters not cease in the presence of the Father’s mercy” (Letters 56:5 [A.D. 253]).
“Atticus, sleep in peace, secure in your safety, and pray anxiously for our sins” (funerary inscription near St. Sabina’s in Rome [A.D. 300]).
“Pray for your parents, Matronata Matrona. She lived one year, fifty-two days” (ibid.).
“Mother of God, [listen to] my petitions; do not disregard us in adversity, but rescue us from danger” (Rylands Papyrus 3 [A.D. 350]).
“Therefore, we pray [ask] you, the most excellent among women, who glories in the confidence of your maternal honors, that you would unceasingly keep us in remembrance. O holy Mother of God, remember us, I say, who make our boast in you, and who in august hymns celebrate the memory, which will ever live, and never fade away”(Oration on Simeon and Anna 14 [A.D. 305])
Cyril of Jerusalem
“Then [during the Eucharistic prayer] we make mention also of those who have already fallen asleep: first, the patriarchs, prophets, apostles, and martyrs, that through their prayers and supplications God would receive our petition . . . ” (Catechetical Lectures 23:9 [A.D. 350]).
Hilary of Poitiers
“To those who wish to stand [in God’s grace], neither the guardianship of saints nor the defenses of angels are wanting” (Commentary on the Psalms 124:5:6 [A.D. 365]).
Ephraim the Syrian
“You victorious martyrs who endured torments gladly for the sake of the God and Savior, you who have boldness of speech toward the Lord himself, you saints, intercede for us who are timid and sinful men, full of sloth, that the grace of Christ may come upon us, and enlighten the hearts of all of us so that we may love him” (Commentary on Mark [A.D. 370]).
The Liturgy of St. Basil
“By the command of your only-begotten Son we communicate with the memory of your saints . . . by whose prayers and supplications have mercy upon us all, and deliver us for the sake of your holy name” (Liturgy of St. Basil [A.D. 373]).
Gregory of Nazianz
“May you [Cyprian] look down from above propitiously upon us, and guide our word and life; and shepherd this sacred flock . . . gladden the Holy Trinity, before which you stand” (Orations 17 [A.D. 380]).
“Yes, I am well assured that [my father’s] intercession is of more avail now than was his instruction in former days, since he is closer to God, now that he has shaken off his bodily fetters, and freed his mind from the clay that obscured it, and holds conversation naked with the nakedness of the prime and purest mind . . . ”
Gregory of Nyssa
“[Ephraim], you who are standing at the divine altar [in heaven] . . . bear us all in remembrance, petitioning for us the remission of sins, and the fruition of an everlasting kingdom” (Sermon on Ephraim the Syrian [A.D. 380]).
“He that wears the purple [i.e., a royal man] . . . stands begging of the saints to be his patrons with God, and he that wears a diadem begs the tentmaker [Paul] and the fisherman [Peter] as patrons, even though they be dead” (Homilies on Second Corinthians 26 [A.D. 392]).
“You (Vigilantius) say in your book that while we live we are able to pray for each other, but afterwards when we have died, the prayer of no person for another can be heard. . . . But if the apostles and martyrs while still in the body can pray for others, at a time when they ought still be solicitous about themselves, how much more will they do so after their crowns, victories, and triumphs?” (Against Vigilantius 6 [A.D. 406]).
“A Christian people celebrates together in religious solemnity the memorials of the martyrs, both to encourage their being imitated and so that it can share in their merits and be aided by their prayers” (Against Faustus the Manichean [A.D. 400]).
“There is an ecclesiastical discipline, as the faithful know, when the names of the martyrs are read aloud in that place at the altar of God, where prayer is not offered for them. Prayer, however, is offered for the dead who are remembered. For it is wrong to pray for a martyr, to whose prayers we ought ourselves be commended” (Sermons 159:1 [A.D. 411]).
“At the Lord’s table we do not commemorate martyrs in the same way that we do others who rest in peace so as to pray for them, but rather that they may pray for us that we may follow in their footsteps” (Homilies on John 84 [A.D. 416]).
From these quotes I hope you can see this practice is very ancient. These comments, except for Jerome’s, speak of the intercession of the Saints matter of factly, indicating its universal acceptance.
The lack of recommendation of this practice in scripture does not prove anything since in John 21:25 the Saint tells us that if all that Jesus did and said were written down the world could not contain the books. This is why the Catholic Church is not restricted only to the written WORD but also accepts what has been passed down to us in the oral WORD.