Vegeta has a whole different degree of control over the oozaru and Gohan was especially talented since the beginning. Ki control comes easier for saiyans than other races, but it isn't necessarily inherent, as both Goku and Gohan had to learn how to do it. Gohan's talent might have come out thanks to the heightened strength of the oozaru, or because it represents a breaking of a limiter. On the other hand, Goku's oozaru form might have been hindered because at the time ki control wasn't such an immediate thing for Goku, so his oozaru self might not be able to go that far, even if it maybe had the potential for doing it. We also have to remember how, narratively speaking, mouth beams weren't a thing yet.
This also isn't anything different than other forms of ki projection to which some characters are used while others aren't. Everyone has the potential to shoot ki from the mouth, like Nappa and Rekoom do, but not everyone does it, like with ki swords, feet kamehameha, volley blasts etc.., it could very well be a matter of ingenuity or will.
All this to say there's no indication that shooting mouth beams is naturally inherent to every oozaru, as it's not exactly physiological and straight forward to assume. A number of factors could be at play and we're given no direct informations, so I feel like it's safer to apply the rule of the thumb.