The thought that if you train in each of those that you’ll understand JKD.
What you learn is boxing, wing chun, and fencing. You don’t learn JKD.
Did these influence Bruce Lee? Absolutely!
Can training in these other arts help you gain more insight into your jkd. Maybe. Depends on how much jkd you’ve trained.
If you’ve only done a few seminars with JKD people and you’ve been taking boxing classes twice a week for a year...you’re a boxer.
Simple answer is because the goals are different.
Boxing restricts to just the hands and is a sport. It doesn’t even have a backfist!
Wing chun is passive. JKD isn’t. The fundamental philosophy is different. Otherwise Bruce Lee would have just kept the name Wing Chun and never created Jeet Kune Do. That’s what he called it when he taught in Seattle and Oakland.
Fencing is a weapon art. And a sport.
What is often left out, though I’ve heard a few JKD people say it, is that all of these were influences and it was Bruce Lee that was the creative force.
Is there benefit in training in these arts? YES!
Firstly, just for their own sake. Each of these are great arts, whether sport or not.
Secondly, cross training is always a good thing to help your own training. As a supplement. You get a different perspective.
Which leads to the third reason it’s good to supplement your training with these arts. They will give you a different perspective and insight to tactics.
Boxing restricts to certain punches. You have to work within a box (pun intended!). You can’t rely on kicks. It also lets you work on timing and distancing in punching range.
Fencing is pretty amazing. It puts all the tactical aspects of a fight into the foil. It also amplifies all of your mistakes. It really lets you put everything under a microscope.
Wing Chun is a great art. It was the art Bruce Lee used as his base. He taught it! How will it help a JKD practitioner? It really shows where Bruce Lee made those modifications, where he took the passive movements out. Sifu Jerry Poteet said that they (Jerry Poteet, Steve Golden, Bob Bremer, Pete Jacobs, Daniel Lee) had been doing a drill at the house for months when Bruce Lee came in one day and said to throw it away. It was too passive.
So in conclusion, will training in Boxing, Fencing, or Wing Chun teach you Jeet Kune Do? No. Are they great for their own sake? Yes! Would they help a Jeet Kune Do practitioner better his/her own JKD training? I don’t know.
Go find out for yourself.