Because the correct minimum set of people is those who are in the channel when it was shared to the channel… and most people aren’t thinking of possible future information leaks when sharing documents directly into a channel with specific people the document is intended for…
The mismatch makes sense to programmers and IT types who have internalised the “computers are stupider than we make them look” way of thinking… but most normal non computer professionals are probably just thinking “I am sharing this with these people right now.