They wont. This isn't the first time it has happened and it won't be the last. They're cheap to install and replace, that's all they care about.
The force of hitting a wall would be very different and the impact would be distributed across your body, but it also gives you the ability to slide along the barrier without having it slice through you.
Michigan State Police posts pics on their Facebook every time one actually works as intended. They showed one that they claim stopped a semi truck, but it was debatable.
The cables act like a knife, or a cheese slicer at speed. You could be doing the speed limit, or less, and still get cut to pieces by them. A solid barrier would be much better to hit, of you had a choice.
Follow @SeenOnWoodward on Twitter to see lots of pics of what's down there. Here were a few from his account.
http://tapatalk.imageshack.com/v2/14/08/16/fdaf1f38bfb6a70de0356195550d034e.jpghttp://tapatalk.imageshack.com/v2/14/08/16/506085b22536f318004733b6a9bdae65.jpghttp://tapatalk.imageshack.com/v2/14/08/16/08c956de45b7c8f57a1239a0e29a3bfe.jpghttp://tapatalk.imageshack.com/v2/14/08/16/2b540cd0d92ce1a9ae2de9d9e001d2ea.jpghttp://tapatalk.imageshack.com/v2/14/08/16/2311c52093e3743a63f926832f5fa116.jpghttp://tapatalk.imageshack.com/v2/14/08/16/7294458a424e97f3a0e99e3fbbdd6da8.jpghttp://tapatalk.imageshack.com/v2/14/08/16/c92d93c0ca65886d5c7e0616433b3be3.jpg
Actually, a lot of people will already be gathering there starting today, I usually walk between 12 Mile and downtown Birmingham, but won't be able to make it this year (again) since my wife's going to have a kid any minute.
Don't try to drive on Woodward on Saturday after about noon. Park in a neighborhood between 11 and 13 Mile and walk.
Chrysler has a parking lot rented at 13 Mile with a bunch of stuff on display and Richard Rawlings will be there. Today they are revealing a "new performance vehicle" down there.
http://jalopnik.com/the-dodge-charger-hellcat-is-almost-certainly-coming-to-1620141202
If you go straight into the cables it doesn't matter much. By the time it slices the C pillar your heads already gone. The link MoJoe posted was a Gen V hardtop.