Why can't kids get effin yoked?

This may be a stupid question, I’m really sorry if I’m accidentally wasting someone’s time. A friend of mine asked me this question recently, and i realized that I don’t actually have an answer.

Why, exactly, is it impossible for young children to build large quantities of muscle mass?

If a very motivated 9 year old starts lifting with high levels of intensity, under the guidance and supervision of a competent, experienced coach, is there any one specific reason BBM would point towards that explains why they wouldn’t put on several dozen pounds of muscle mass and add 150+ pounds to each of their main lifts within 3-4 years?

Like, if a 9 year old begins training, and on the first day they are about 80 pounds and well under 5 feet tall, deadlifting less than 75 pounds and they eat 120g of protein each day and do sensible, correctly programmed barbell training for three consecutive years… why would they not make the same progress as an adult would under those circumstances? Why would you expect an individual who goes through this process to not have the same level of development at age 12 and a height of 5’0 that an adult who’s height is 5’0 would have? The amateurish, pop-sci answer to this probably has to do with hormonal milieu - but would an adult woman have significantly more testosterone per ng/dl than the average 10 y/o boy? Is the reason kids aren’t getting shredded that they can’t focus on it/don’t have access to top-level programming, or is it something inherent in the experience of kidness?

Thanks.

They don’t generate the same hypertrophy response as their post-pubertal counterparts, which blunts how much muscle mass they can attain. This is due to all sorts of things including their physiological inability to create some fatigue products, hormonal milieu, motor unit recruitment patterns, etc.