Like playing music. Jamming is awesome but if you have a regular group, you start to know the same songs and the opportunity to get deeper in to the music and take it interesting places opens up. The waza are the songs we all know, playing around within the structure of the waza is the fun part imo. You don't graduate from art. You learn the techniques and then you play with them. It carries into the other parts of your life and you see the world through that lens.
Brian, that's interesting you mention music. I took lessons on multiple instruments, and participated bands. I also took a recording classes, and interned at a recording studio. I haven't been a student in any formal class setting involving music or recording since the mid 80's - which overall had me taking about 10 years of various kinds of instruction prior to that -with, in most cases, highly-qualified instructors. And even in those ten years, I often freely got together with other musicians outside of any structured class setting - and it's probably during those times that I actually deepened my understanding most of the craft and art of music.
I agree that we don't graduate from art. But we do go through levels of understanding and abilities within the craft of whatever artform we're expressing ourselves. Learning the craft is like the shu stage of learning - under others more advanced. Writing, aikido, music, etc. all have a craft within the art. And before the art can truly be expressed, the craft needs to be mastered to a degree. A commonly accepted time frame for mastery is 10,000 hours. I tend to think that's about right.
In aikido, shodan gets people to the basic level of understanding of techniques. Nidan focuses more on pulling all that together in application. Sandan marks what is really the end of learning the "craft" aspect of aikido. Yondan is the beginning of the expression of the artform - and entering the ha stage. Godan more so - with hints of the ri stage beginning to come into the picture. Rokudan and up is all ri. Of course there is shu within ha, and shu within ri, and ha within shu, and ri within shu...
Dan, are you speaking from an extensive background in skateboarding, skiing, tennis and et cetera? How do you know that people in these activities are "left to their own devices"? For that matter, how do you know that they don't have ranking systems?
I can speak from an extensive background in numerous fields.
And while there are not "ranks" per se, I would agree that there are levels. Just as there are levels within martial arts, regardless of whether one is ranked.
If all the endeavors and passions of mine required that I entered a classroom with an instructor - in perpetuity - in order to participate, progress, and enjoy the activities - I would have dropped them long ago.
In most endeavors, studies, and activities the "structure" does not come from a self-imposed outer hierarchy. The structure, learning, progress, and enjoyment - and sometimes toil and sweat - is contained within the activity itself.