They already exist here. The restrictions only seemed to be on compounding pharmacies. But the US has "med-spa" and "rejuvenation center" chains that offer them; I'm guessing at significant markup compared to ordering from random chat-app or website. Since it was a "grey" area who knows if those centers are more trustworthy about their supply sources.
doubt current supplement labs would be up to the task of certifying things injectable
very risky to bypass gut digestion safety in the human body with unknowns
Also US MedSpas and other boutique pseudo not-quite-medical clinics will already inject you with just about any peptide other than insulin as well as NAD+ if you feel you need it for some reason. You don't, but they'll still do it.
Is their plan really to just keep injecting themselves with amino acids all day every day?
Why not something like an mRNA injection so you just produce more of whatever amino acids or proteins are supposedly "Needed"?
This feels more like Vitamins are played out for the supplements industry and they need a new market with vague benefits to milk.
There are also peptides that seem to have similar effects as regular exercise; a weekly injection might give you the same benefits as a daily mile run. These haven't been well studied, however, and we don't understand their safety profiles.
But the influencer set has taken these possibilities and run with them, because they're more interested in clicks and views that science and facts.