"Being an adult" is only as hard as you make it.
I think most parents are content to let their kids flounder on their own. I was told what I did wrong, but most often no one told me how to do it right. We learn by example, but you can't learn a thing by watching your parents balance a checkbook. Lessons need to be passed along. If they aren't we are all just running in circles.
A child is only a child for one fifth of their lives. If they are not taught adult things, by the time they grow up they are unarmed. It's easy to make a blanket statement, but not everyone is equipped the same. Some have better opportunities. Some don't know how to recognize an opportunity.
The type of person you are has a lot to do with being an adult. A timid or uncertain person isn't going as far as a bold and aggressive person. An empathetic person won't take advantage of others the way an apathetic person can climb to the top. An abused child isn't going to put themselves out there the same as a nurtured child will.
Society doesn't help you to be an adult. Society can smack you down quicker than you can get back up. There is always someone that wants to push you aside for their own benefit. There is always someone to take what you've got. You will always be taken advantage of by someone.
How do we even define, "being an adult?"
Sure, there comes a point where you are responsible for where you are, but if you've not been given the tools how could you have gotten further? More often than not, making a mistake just leads you to another mistake. Eventually you'll get it right, and then you move on to the next mistake. It comes down to how quickly you get things right.
I'll oppose your sentiment. It's not that it's as hard as you make it. It's that it is not as easy as if you'd been prepped for it.
You hop the hurdles before you, but some have higher hurdles than others. If people were less quick to condemn and more eager to assist being an adult wouldn't be hard at all.
It's just life. Sometimes it sucks.