It's not, Human growth hormone (HGH) doesn't promote growth directly. Instead, it encourages the liver to release insulin-like growth factor 1 (IGF-1), which thickens and elongates bones. And currently, there is no IGF-1 hormone, only synthetic replicates which do NOT work
The current synthetic way is these and none of them works:
GF-1 DES barely binds to all IGFBPs (these proteins tell IGF-1 in the blood where to go, regulate bioavailability and half life), so it'll just immediately attach to IGF-1 receptors near the injection site and not really go anywhere else.
IGF-1 LR3 doesn't really bind well to certain IGFBPs like IGFBP-3, so it basically won't really be transported to growth plates or other sources of long bone cartilage.
Receptor Grade IGF-1 LR3 is basically a more pure and slightly enhanced IGF-1 LR3, but since it's LR3, it can't really bind to certain IGFBPs like IGFBP-3.
Heightmaxing is very complicated, and I'm very convinced science doesn't understand it yet.