True, he is an adult and as such, he can find his own path, but it is hard and frustrating. Think about it: he spent the majority of his life just having fun and not thinking about building his own career. He never had to think about what type of career he'd work toward. He was never taught even worry about his future. Kris neglected to guide him in this regard, but she made sure to guide her daughters.
Most kids growing up in his position would love that carefree, lazy, fun-filled, no-worry life. But eventually, when you become a young adult reality sets in and you want to have your own. But so many important years have passed Rob without the needed guidance and training for him to do for himself. Now, he's struggling to begin finding his path to success at such a late start in life. Though it can be done, I'm sure its frustrating -- especially when he still sees how much time and effort his mother continues to invest in his sisters careers but not in helping him. He sees them all over the media and his mother promoting them. But then he just keeps hearing them accuse him of just being lazy and telling him to just go out and get his own career -- which non of them ever had to do for themselves.
So, I completely understand his frustration.