I don't see it for preschool, but kindergarten was always a milestone as far as I am concerned. It's when they start "real" school and are no longer the little kids. Their days are no longer filled with all play, all about me, all day long. Granted, most kids get some direction to focus outside of themselves, but the kids still spend the majority of their time focusing on fun things or entertaining things. Even the learning in kindergarten is mostly teaching skills in as fun a way as possible. After that, when they start 1st grade, the focus is much less on the fun aspect, and much more on the necessity of learning the skill itself. It changes more into "work" than "play".
Sort of like when they finish high school, they move on into more mature life. Either college, or a job...
Husband and I had an interesting discussion on a related topic just a few days ago. His position was that he doesn't quite "get" the full out celebrations of adults going back to school and getting a master's degree. Not that it isn't worthy of celebrating or a worthy thing to do. Not at all. But that it just seems substantially less "momentous" than graduating high school or college.
In our discussions, we pretty much came to consensus that it is because it isn't a milestone moment in life. Graduating high school or college, (or getting a master's immediately after college without taking a break in school) is a major life change. Folks move from being a "student" to being an actual productive member of society (hopefully). Their life changes dramatically... regardless of what they do for a living or what their education is geared towards. They go from being a sponge/learner, to being a DO-er/earner.
These changes tend to no apply to non-traditional students. You are married, with a kid or two, you've worked (or been a SAHparent) and then you go back to school. You have already made the change from "student" status to things much more important. As a typical high school or college student, graduating IS your most important accomplishment in your life. Really, it is. But if you've already been married and have kids and are a parent, finishing school (simultaneously) may be harder than doing it when you could solely focus on yourself and your studies, but it fails to qualify as the most important accomplishment in your life anymore. Right? I know it would for me. My biggest accomplishments will always be my successful marriage and my kids, not a degree, no matter how difficult to achieve it might be.
So while technically the accomplishment is the same (or maybe even bigger/harder?), I don't celebrate it the same or get quite as excited as for someone making a total life change from "student" to "working adult"....
So, I guess I see preschool/kindergarten graduations in the same light. Preschool? Mehh... they aren't really finished doing anything. They still have kindergarten... and while kindergarten is a lot more academically focused than it used to be, it is still kindergarten--which is a lot of fun and play. Graduating kindergarten, is a big change in the life of a child who is about to start grade school.
It qualifies as a milestone in my book. Preschool, not so much.