One of the reasons has to be that their very livelihood depends on it @RoseHarper.
So they can continue to feed us the lie that we are getting a peek into their ‘normal life’ reality. They rely on the ‘reality’ of their content to engage followers. Engaged followers equals brand endorsement (goods, services, cash) and book deals.
Posts marked AD get much lower engagement (which is why we see they begging to ‘check out the grid’ post on stories). If it’s all or largely marked AD correctly as per CMA guidelines, with low engagement they can’t sell themselves as efficiently to brands and pr companies.