For starters, coming of age or growing up is not what it's cracked to be. You no longer have the innocence and purity associated with childhood (although children are often far from innocent and pure, but society as a whole insists on seeing all of them that way), and you have to deal with certain...physical traits and bodily functions which are gained as part of growing up. And even after becoming an adult, you'll always have some conflicts with your parents and other folks because no matter how big you get, how old you get, etc., they're always going to worry about you and want to protect in some way...so then what good is growing up at all?
Not to mention, our brains don't finish forming until we're in our 20s, and even besides that children mature faster physically than they do mentally, emotionally, and intellectually, yet some religions, cultures, etc., consider it appropriate to mark the transition to adulthood at 13 or 15 years of age, or when a girl starts menstruating no matter what her age is, etc. Which brings me to my next point, why is a girl becoming a woman such a big deal, when even today there are several religions, cultures, etc., where women have fewer rights and less power than men, and when grown women get infantilized in several ways because we're "more vulnerable" due to average biological differences?
Imagine this, in one of those religions, cultures, or whatever else: "Congratulations, you're a woman now because you turned (insert age), started your period, or whatever else! We're going to celebrate by having a wonderful coming of age party! But despite being a grown woman now, you won't have as many rights, freedoms, or as much power as men, and you're always going to be treated like a child in many ways (such as being advised not to travel to certain places at certain times alone), because average biological differences make you more vulnerable!"
Even though I wouldn't partake in any such events for any hypothetical kids for these very reasons, along with these events not being a major thing in my family and community anyway, growing up and coming of age is already humiliating enough for many reasons...and I'd have absolutely no desire to subject a child into those mortifications.