Because human creatures gain insight by hearing a given concept from others. I think Scripture says it this way:
“Folly is joy to him that is destitute of wisdom: but a man of understanding walketh uprightly. Without counsel purposes are disappointed: but in the multitude of counsellors they are established.”
Because no man is an island unto himself, we each need other saints to help us see where we have blind spots, etc. Regarding the witness to the world, preaching the cross to spiritually dead people is the only thing we can do - it’s what the Scripture says for us to do. Yet the words and phrases that one says may be better understood than what the other says - and either one is fine if it is biblical and not a story of man’s creation.
This is why there are 4 Gospel letters in the Bible.
Who says we are qualified to preach and by whose authority?