It comes down to prospects. Like a lot of pinball designers, I have a background in Industrial Design and I've been working in a creative role in a manufacturing-based industry for almost 15 years. Industrial designers can apply their talents within many different fields from automotive design to everyday consumer products and toys. One of the key things I looked for when I got out of school was a field that had a future. Pinball has not exactly been a business with great prospects for many years.
Things may be turning around a bit, but sadly, unless a student was absolutely drawn to the pinball industry right from the start, it's doubtful they would end up there on purpose. Companies like Stern would do well to sponsor ID competitions and other grass roots intititives with local college design programs and offer students an introduction to pinball as a career.
I'm 37 and actually toyed with this idea for years, I may even attend JPOPs "pinball school" if he gets it going, but with a family and a mortgage, the only way I'd get into designing pins would be as an entrepreneur/owner, not as an employee. Without mentors or support, this would be a very hard road to take. Also, mentoring is a big deal and involves a lot of investment. Not sure many of these older designers would be up for it when there are precious few jobs in the industry for them as it is.