What is it that makes Parker Valby so injury prone?

No it isn't. It's not even close.

Valby was basically running full gas every race. When you look at it that way, she doesn't seem to have a big problem with durability.

Weak/thin bones or a lack of strength and doing too much too soon? I'm confused as to how you couldn't reach that conclusion yourself

She’s petite.

With regards to swimming, I think there is something to that. Not that there is a tendency for the best swimmers to have low bone density...that's just a consequence of training for a non weight bearing sport. However I do think that well trained swimmers (many swim for 2+ hours a day, 6000+ yards per day) have an amazing aerobic engine. You can just about go hard in swimming every day, unlike running. Often times swimmers can come into cross country in 9th grade with the cardiovascular ability to go much harder and longer than their bones, ligaments/tendons and muscles are prepared for. Unlike the other sport that transfers well to running....soccer. It would be interesting to see if there tends to be more injuries in swimmer/runners versus soccer player/runners. That's where it's imperative that a coach makes sure to slowly progress his/her new runners and not just put them in groups based on times. As a swimmer before turning runner myself, I ended up with a stress fracture in my 9th grade cross country season.

As far as Valby is concerned, she's been a runner long enough that I don't think the fact that she was originally a swimmer has anything to do with injuries occurring recently. I know she did have bone issues to begin with, but it seems like the latest struggles have to do with a nerve issue. Hopefully her medical team has been able to get to the bottom of that issue. My best guess is that she's found a training regiment that is working for her and she doesn't want to mess with what appears to be successful.

I don't want to get into the realm of ED/anorexia or anything like this for disclaimer.

But she is definitely on the skinnier side all year round. If you're training extremely hard whilst very light weight and not eating enough you pick up a lot more injuries and can't recover as quick.

I'm guessing this could potentially be why she gets injured so often from running, because her bone density is very low and body is generally just not durable due to her current body type. She can get away with using the arc trainer due to the lack of impact/stress on the body. But even then I'd say she is likely fuelling at a very risky level.
The + of this is that she's at race weight basically 24/7, but imo it would be impossible to run a lot at race weight and stay totally healthy. In high school she looked to be a bit bigger, which is probably why she was able to run a lot more miles.

The double-standard is strong here. Every kind of conjecture is fair comment on threads about African runners but not American runners - and especially female runners with a sycophantic fan base. But it is amusing that posts that provoke a reaction amongst fans are deleted by partisan mods, leaving the fans posts up effectively arguing with nobody.

So how can she achieve leaps forward on less training than her competitors, and results that others who cross-train don't get?

Devils advocate: Does she get injured a lot? Do we know that? Or, did she get a couple of stress fractures and realized the X training worked for her and is playing it safe? Maybe a mature, and clearly successful, decision. What injures do we know about?

Is she ready to open up a can wupp butt on everyone once pro.

She's not doing less training. She's likely doing more training time-wise than they are.

About sixteen or seventeen years I wanted to set lifetime weight lifting personal bests. I wanted to make weight training my primary activity. I intentionally went up to (215 to 220) lbs. I ran no more than 7 miles per week. I did (10 to 12) hours per week on gym endurance equipment, usually elliptical or stationary bike. Spring arrived, I cancelled my gym membership and upped my weekly mileage to (12 to 15) miles per week. Six months of running (12 to 15) miles per week with no gym membership, I lost fitness from not doing (10 to 12) hours per week on gym endurance equipment. Try running three days per week plus 10 hours on gym endurance equipment. Just try it.

You are just speculating and do not know that. Allie O did 2 hours a day cross training when she was injured in school.

All we know is what the ahletes tell us. Based on what they have said, Valby puts in more total time of cardio.

Likely? Based on what? Will she be running the equivalent of 70-80mpw on a trainer? BTW, that's distance, not time.

You're not describing training to excel as a distance runner.

Who else is training this way as a distance runner and getting her kind of results?

No one. Which is why it’s so intriguing that this approach can possibly work for others.