Where else could it have come from? Judaism nor Islam venerate saints like this so it seems like something uniquely Christian.
I know there is also a semantic difference in how Catholics pray to the saint to then pray on their behalf to God instead of directly to the saint and the saints thrmselves aren’t technically considered deities despite their supernatural feats, but those don’t really seem like functional differences.
This was asked earlier this month and one of the responses linked this excellent explanation from /u/thejukeboxhero.
I guess I have some comments and want to seek clarifications on your initial query.
That difference isn’t just semantic. The idea is that the supernatural feats were gifts and blessings from God rather than inherent to the saints themselves; the inherent elements are considered to be the lives of holiness and self-sacrifice that they led, commonly facing suffering or death for their faith. The above linked post details the “why” of this very well
Further, “veneration” (formally called dulia) is considered distinct from “adoration,” (formally called latria) which is something reserved for only God (explanation of the distinction can be found in the writings of Aquinas). While you refer to this as a semantic difference, it really is more about intent being lost in translation from other languages.
However, many Protestants do often compare this practice to idolatry; it’s a major sticking point to this day in discussions between the various Christian denominations.
Finally, there was already a long tradition of honoring those who were killed or suffered for their faith (see “Martyrdom of Polycarp” from the early/mid 2nd century).
Also - What is your source on Islam not venerating individuals? I’ve seen that certain branches do in fact have a similar practice, though it has diminished of late. I don’t know much about that other than that it exists or existed, so I suppose I’m seeking an understanding of your statement.
I want to chime in here to recommend the work of Peter Brown, specifically his excellent book The Cult of the Saints. He connects the idea of saintly patrons less with patron deities and more with earthly patronage systems that existed in the Late Empire. Early Christians felt a strong connection to the dead and saw the very holy dead as potential patrons to exert influence for them in Heaven, much as Roman patrons exerted influence in the land of the living. This is a crucial element to understanding how the cult of the saints developed and the way it formed how it did. It wasn't a given, seeing as there was precious like it in Judaism. But Brown's work -- and the work in response to it on Late Antique Christianity -- really shows how the development came from Roman social and political norms.