SaiyajinPerfection

The Kamehameha is one of the few attacks out of the hundreds of attacks in Dragon Ball where the name of the attack is actually said. For example, there are tons of attacks in Dragon Ball that don't have verberated names, like Freeza's Death Ball and Majin Buu's "fat pull entraping" technique, Even Goku's Shunkan Ido ("Instant Transmission").

But of course shouting the name isn't nessicary, Goten says Kame-kame-ha and it still results in the same attack. Buu also doesn't need to say it in order to perform it correctly. From and in-universe perpspective, it's probably easier, because it then reminds the user what attack they want to use instead of it just coming out as a random stream of ki. From an out-of-universe perspective, it's probably just to show the viewer or reader what attack the character is about to use. Most of the manga is in black and white anyway, it's hard to tell what attack they're using sometimes.

And their are several canon variations of the Kamehameha anyway, and usually the same name is shouted. For example, Goku doesn't say "Shunkan Ido Kamehameha", it's just Kamehama. I can't even remember if he used the name during the time that he shot a kamehameha from his feet and lunged at Piccolo in Dragon Ball.

Even in DBZ, Goku uses a variation only seen once. During his battle with Freeza, Goku is underwater and mentally says "Kamehameha", creating two stationary masses of ki; which he then launches at Freeza moments later to distract him. He didn't actually verberate "Kamehameha" since he was underwater (DB: vol 26/DBZ 10, p. 124)

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