Terra met Riku, Aqua met Kairi, Ven met Sora. Vens heart joins with Soras at the conclusion of his game. Aqua recalls Kairi in the world of dakrness when she grips her charm, Terra sees the KH2 Riku when he sees Riku as a child. Thus, Terra gave the ablitiy to wield the keyblade to Riku, Aqua gave the ability to wield the keyblade to Kairi, and since Vens heart joined with Sora, Sora also gained the ability to wield the keyblade, And also why he was able to overcome the darkness when it took over Destinys Islands in KH1, Because Ven was still in Sora, And when Sora unlocked his heart in KH1 to release Kairis heart from within Him, Vens heart left, And when the nobody Roxas was formed, Vens heart went inside Roxas, explaining Why Roxas looks like Ven and thinks he has a heart.

There's a cutscene where Terra lets Riku hold his keyblade.

So I assume Aqua did the same, and Ven joined sora so that explains why the yhave keyblades

 

MOD EDIT: Don't tripple post. Edit the first one.

I know this is a random late reply to this topic, but idgaf.

If you gain the ability to wield a Keyblade just by being given it to hold, I'm expecting to see Jack Sparrow kicking some major ass in KH3, personally.

 

Rite of Succession busts that theory. Sora's not a master, and even then, he didn't hold the Keyblade out to Jack like Terra did to Riku, both holding it at the same time. Jack was able to hold it for a while because Sora had willed the Keyblade to let him do so, just to humor him until he called it back.