Base movement speed

Can someone explain to me why melee champions that have high mobility contained within their kits, also have high base movement speed? I am curious as to why this is.

Champions like Lee Sin, Yasuo, Zed, have a base of 320 or more, and the AD assassins more than often get more than 1 movement speed item. Phantom Dancer, and Ghostblade, and they have 500 MS temporarily, but 410-420 passively. You fight them and they just run away, you lose the fight, and you can't get away. It effectively gives them a get out of jail free card if they waste their mobility.

When you look at champions like Ahri, She can't waste her mobility spells, because after she does you just go up to her and beat her face in.

Anyone shed some light on it?