I've seen a couple (long time ago) of ballistic gel tests with Trad equipment.
You ask why you don't see it used with trad equipment.
Answer:
(a) Ballistic gel is expensive, and/or a pain in the butt to make it yourself.
(b) It doesn't produce anymore definitive results, than a lot of other things used. Not that I've seen anyway.
Nothing totally resembles, or can replace the consistency & density of an animal's tissue.
But, resistance is resistance, and penetration by force is just that, so an arrow penetrating "the best" in (lets say a foam block) will most assuredly penetrate "the best" in an animal.
Of course, you then have to take into consideration the design of the broadhead used (it's cutting efficiency), and the structural integrity of both it, and the arrow used, and combine those with the arrow having "the best penetration by force". There again "the best" will be "the best", regardless of what you are shooting it into, including an animal.
It may not penetrate "the same" in the animal as it does in the block, but "the best" one in the block will still be "the best" one in an animal.
Get you some 1/2" plywood, and glue a thick (at least 16") foam block onto the back of it. Go shoot it. You'll be taxing both the penetration power of that arrow as well as it & the broadheads structural integrity.
After shooting it several times with different configurations, and taking an average of each - Whatever arrow configuration you find works "the best" in it, will also be the one to work "the best" in an animal.
Obviously, that's just my opinion, but it's my opinion based on exhaustive testing over the course of many years, shooting many MANY different arrow configurations, and shooting them into many different types of target mediums. What I found to be "the best" in those mediums has always wound up being "the best" (for me) in the animals also.
That said: Anyone willing to provide me with a couple of ballistic gel blocks, that are adequate for doing the testing, I would be more than happy to do it. A couple of 18"x18"x24" should do it.
Rick