They do all these campaigns for fat women to celebrate their bodies so why not do a campaign for flat chested petite girls to celebrate their bodies too?

Because they are hypocrites. Apparently you can´t like petite flat chested girls only unhealthy fat old women according to the feminist narrative.

There OnlyFans flat chested girls who are double AA, like kristi_lei, lilyoneill, dreamlilo etc who are of course +18 only that 100% or 90% flat chested and as per OnlyFans and Reddit rules they are not allowed to advertise themselves as looking underage or posing in child settings, they never do that but who do they want to deceit, who is going to subscribe to porn pics of a girl who has zero tits? And they have thousands of subscribers in OF.

And Japanese girls too, Asians are smaller in size than Westerners, an 18yo Japanese girl with small tits is going to attract people who like petite girls with the added advantage that Asian girls rarely have tattoos but in this hypocritical society we will never see a campaign for women who look younger than their age to celebrate it. As the article you posted proves, the campaign is for girls who look too young to be ashamed of it and force themselves to look older.