Famed Honky Gary Trudeau, creator of the Doonesbury comic strip, once ran a series making fun of Gladys Knight's backup singers, the Pips. To his eyes and ears, they weren't doing much back there, just moving a little and singing "Ooohh" at the end.
Well, a lot of white people think that, but they should shut up when they don't know what they're talking about. We grew up thinking that the tune and the words were just about all there was to a song. There might be arrangements of a song, but really, same thing. People who studied music started to grasp more about dynamics in music, but the requirement in chorus or choir was pretty much always the same for white people. Sing the note true, with no flourishes; keep the rhythm exact, except maybe some little expressive delays if you were the lead singer; enunciate the words clearly, because there were lots and lots of them, and they'd get lost if you weren't precise. It was a style of music made more possible by the printing press, plus the increasing affluence which allowed people to buy instruments.
They didn't have 20-piece orchestras in the slave quarters, remember? In black music, the voice is used as an instrument. The dynamics are more intense, the flourishes and bent notes much more common. Here's a fairly extreme example.
There were white traditions, most notably Church of Christ congregations that forbade instruments not mentioned in the New Testament. Which is like, everything. But the developed style was primarily African-American.
How things get their start is not necessarily why they perpetuate. Even after slavery, even as black musicians obtained and mastered instruments, the half-voice/half-instrument style persisted and developed. Blues guitar includes a lot of backup instrumentation now, but if you listen to older blues artists, you will hear the guitar or harmonica taking on vocal characteristics, the human voices carrying some imitation of the accompanying instruments, including percussion. As these were often songs for performance, not just recording, the audience or congregation is often brought in to sing bits, providing more instruments. Here are two from the 20's-40's that illustrate this.
Sam Cook later sang with the Soul Stirrers, BTW.
When you do this, the clarity of the words gives way. No six verses of complex poetry for this style. So to get the message across, you have to repeat the words many times. To white people, this seems irritating. Why keep saying the same thing over and over? Isn't that a little theologically oversimplified? Well, maybe. But when you've got whole congregations, whole cultures that are skilled in using dynamics and flourishes, they're going to hear things beyond just the melody and the words. And they're going to be able to do things others can't, melding instrumental and vocal into a different sort of symphony.