I am glad you posted this.
I know since 2009 there was a letter sent to Sport Rider (IIRC...or some other sportbike magazine) of a rider asking the exact same question you posed...how come there is no neck protection for street motorcycle riding but there is for offroad/dirt. The answer to the letter said that Alpinestars was developing a system but the answerer to the letter state something along the lines that rarely does someone crash head first (or something like that...I'll have to see if I can find)
What I can tell you is ever since I read that back in 2009 it peaked my interest.
I was living with my parents at the time and my neighbor who rides both street and dirt (mostly dirt) got in a conversation with me about trackdays. He asked if there was any neck protection and I told him not that I know of. He told me I could borrow one of his neck braces he uses for motorcross for track. When I tried it on (can't recall what brand it was he had) the back portion of the brace got in the way of my racesuit hump and was useless. I tried it with my jacket and the design of it was hindering my head movement for visibility.
Time passed and then I heard Leatt started developing one specifically for road use. Leatt is widely known in the MX world for their neck braces. I waited with baited breathe to get my hands on one.
However the airbag suits and vest stole my thunder. I bought an airbag vest from Safer Moto and tried it out on the track. I was not happy with it at all because the CO2 catridge would get caught on the upper lip of my gas tank and really hindered my body position when I used it with my D675 track bike. My girlfriend on the other hand liked hers on her SV650
I know of a handful of riders that actually use the airbag suits/vests...including my girlfriend with great success. Unfortunately...one of my friends who has an airbag vest that has used it before...her most recent crash the airbag vest did not deploy in her last crash. Her last crash was a fast one. FORTUNATELY, she was not hurt.
That right there got me kinda thinking about "What if" the airbag does not deploy like how it didn't deploy for my friends. To this day I am still trying to figure out why....the safety leash pulled with a lot of force...maybe air slowly leaked out of the CO2 cartridge because it didn't puncture and seal properly with the airbag vest mechanism? Maybe a dud CO2 cartridge. Regardless, it was enough for me to think of another alternative.
Finally, about a couple months ago I got my hands on a Leatt STX brace with the primary use for track. It took a good amount of time to get it to fit properly (best to fit it with the gear you will be racing/riding in)..but if you want it to work right you need to make it fit right...this isn't a rush job.
Not only does this brace minimize neck injuries, but also the common broken collarbone due to your own helmet injury also. I know immediately sold.
I got to test it out finally at my first race last month. I am very satisfied with it and it doesnt interfere with my field of vision when in a race position on my Ninja 250.
I got to do some endurance testing on the track putting around 10-15 laps to see if somehow fatigue with it on would creep up. Nope...I don't even notice I have the brace on at all.
Realizing that I would more likely be more prone to a neck injury on the street than the track in the event of an off, I decided to start wearing my Leatt STX neck brace on the street also. I commute everyday to work on my bike.
So my Leatt STX neck brace gets HEAVY use.
Im not sure if the OP posted the same video, but Leatt actually came up with a 17 minute video that scientifically explains their neck brace technology. If I can find it I will post it up.
If ANYONE has any questions about the Leatt STX brace, PLEASE don't hesitate to ask me or PM me. I encourage to post on here since I think its worth public knowledge/opinion
EDIT: Here is the educational/scientific video about the Leatt Neck Braces:
https://www.youtube.com/watch?v=FD6-NDswsdk