Voice acting is acting. It takes talent and skill to do it right, otherwise it sounds bland.

Think of it as your classmates in school reading aloud. It sounds flat, boring, and so on. There's no emotion. Voice acting is acting, as the name implies. All 'normal' actors have voice training so as to convey emotion in their speech. Otherwise it'd all be left up to facial expression and body movement. Not very convincing that way.