Size matters.
Reach matters, strength/weight matters.
You'd get a visceral (ie., non speculative, non thereotical) feeling for the difference in power when you're kicked by a 200 lb opponent vs a 150 lb opponent assuming both are equally skilled. And that's not even remotely close to a 40 to 45 lb halfling.
The bigger the gap in size the more skill you need just to neutralize the size advantage much less gain an advantage.
That's why fighting tournaments have weight classes.
Action movies tend to portray a smaller protagonist having a disadvantage against equally skilled larger opponents, and they tend to win thru luck and wits, and that "makes sense" even by action movie standards.
In fantasy, D&D just pretends that size doesn't matter.
You can argue that a halfling fighter is fast, but equally so are many medium-sized fighter and rogue PCs. You can argue that a halfling fighter is clever, but so are many martial and rogue PCs. In order to neutralize the size advantage, halflings have to be significantly faster and smarter than equally skilled larger counterparts. I see no reason why that's the case; halflings don't even get a +2 Dex in the playtest or higher maximum Dex.
Give halflings a divine boon of Yolanda's Fortune or a vorpal blade or something, and suddenly you have an in-game reason to neutralize the difference.
In your game, you probably won't apply such stringent verisimilitude to your fantasy stories. And that's totally OK, who am I to insist otherwise? But when someone asks "Why?" they generally aren't meaning to ask "Why in your game?" They usually mean to ask "Why should I want it in my game?" [fluff-wise] Replies about crunch and replies like "why not?" don't actually acknowledge and address the specific intent of the question, is all I'm saying.
(Edit: The post below for example is a crunch answer to a fluff question)
