Earlier quoted context omitted.
Because you have to get someone all the way to an offer until you know what their salary expectations are. I think it's a good rule, but it is harder for managers. Especially managers who want to find a cheap hire.
There isn't anything stopping you from asking expectations. Just history, which removes some of your leverage (arguably a good thing). And people looking for "cheap hires" deserve to be hit hard by this the most IMHO. Leveling the playing field with information asymmetry is a good thing.
One could argue that the potential employer is in a position of power and should therefore be constrained but I don't see that as the case at all these days, at least not in the tech industry.