What makes the direct statement more "effective"? It makes it more forceful, to be sure, for exactly the reason you state:

with indirect requests or criticism you run the risk of the recipient having a sense that your criticism is somehow optional, that since you're not stating it forcefully they must have a choice whether or not to make the change

That's exactly the point. They DO have a choice. The other person has agency, and it's their idea. When you state your criticism directly, implying they must do it the way you state, you're expressing that you have greater power or status than they do and this entitles you to direct them to do things differently. This isn't true unless you are actually their boss, and whether you are or not it's belittling and rude; this is why it puts people on the defensive.