Can a 16 year old magically drive a car properly, but a 15 year old can't? Is an 18 year old magically much more capable doing their electoral civic duty than a 17 year old? Is a 21 year old magically able to consume alcohol responsibly, but a 20 year old isn't?
(Or whatever age cutoffs are appropriate for your jurisdiction.)
We define these cutoffs not because they are magical or apply equally to everyone, but because we have to draw the line somewhere, in cases where we aren't going to do a blanket all-ages ban. Sometimes the cutoff is chosen poorly, certainly, but that's a problem with the implementation, not the idea itself.
The actual reasons is that they hope to have captured the childs' reward system by then. Laura Cress must write articles for the BBC if she stopped she would lose her purpose in life and be forced into rehab, she would experience ego death and ostracization until she builds another system approved skill. Current society is heading off a demographic collapse due to this built up debt.
The real problem is that we have invented a society that is less rewarding than a slot machine, not that humans are somehow built wrong. A slot machine or hard drug can only effectively hack ones physiology, a social system can hack the whole stack at once (Physiology, Emotions, Ego, Social belonging). You can give bad actors the pains of withdrawal, peril, existential crisis and social suicide all in one. There are examples throughout very recent history of each layer being captured more perfectly. Even physiology more perfectly than any drug, think enclosure act, 14 hour workdays in industrial England.
Fine, ban lootboxes, but don't pretend it's to protect youths, it's to utilize "children". society is a massively harmful and evil tool, we must acknowledge that it's pure unadulterated evil that wouldn't blink at killing all youths. This is a fact, not an opinion, morals are just an API for humans that the system uses.
However, we can't set up a force of psychoanalysts to assess every member of society and run chmod on them, so we go with a compromise.
This same reasoning applies to sex consent, voting, driving, working.
We want to say "only qualified people can do x" but it's impossible to encode this in regulations and it always boils down to the sorites paradox.
So as a culture we have defaulted to "age is a good a proxy for being qualified".