Because instead of vaguely describing what I did I can show you exactly what I did.

Instead of saying "we normalized the counts", I can show you EXACTLY what that was that I did.

If I can't see your code I don't trust you.

I think it's cleaner, more honest, more reproducible, and it helps teach younger researchers.

ps. Huge amounts of "human" data are normally public and available for anyone to work with, it's only specific subsets that need to be private.