A lot of activity in this thread, more than I expected.
Tagxy addressed this previously, but I'd like to make my own point on this.
Shields in Smash 4 (and Brawl to an extreme) are created this way because characters who need to rush down but get past spacing (Mario, Ganondorf, Samus, etc.) need a method to play safe against characters who are quicker or have disjointed range. One of Mario's best approach options is actually shield-dashing opponents who aim to use their disjointed range against him, and the reason why this is important to know is because without that inherent shield advantage (i.e, high shieldstun and high shield pushback), characters like Mario, Ganondorf, etc. wouldn't have the ability to do anything to cover their natural disadvantages. Characters with longer range would constantly just poke their shield over and over again until they couldn't do anything.
Are there ways to work around this? Yes, though even in Melee's scene, Marth's superior range invalidated a lot of the cast, so Melee's superior speed and movement options doesn't quite help out characters when they have no way to get inside. I should also point out that Marth's superior range and aerial movement in Brawl also invalidated a good majority of the cast as well (and Metaknight to a more extreme effect).
What isn't being realized is that due to free movement and disjointed range being prevalent, having high shield stun would actually hurt a lot of the cast who can't take advantage of this, if only because not every character can have the luxury having powerful options in footsies.
(I'm too sleepy to make a huge post. **** work.)