I don't doubt that you personally had a bad experience getting SSDI. But look at the larger picture:
And then ask yourself: How far do you have to go to find an ad for a firm helping someone receive SSDI? Isn't it on every other billboard? What about TV commercials? They are everywhere. And radio ads? Yup, more SSDI ads there too.
So are we supposed to believe that thousands of these SSDI specialty firms are tricking millions of people to give them money (or a percentage of their benefit) all to no avail?
Again, I don't doubt you had a bad experience. But in the macroeconomic sense, the floodgates are wide open.
It's in my nature to find reasons for observable phenomenon. Something like the obvious increase in SSD enrollment as illustrated by the chart you provided made me curious. Made the government curious too. A report was generated last year addressing that very issue. It's a PDF, the most pertinent information starts on page 6.
http://1.usa.gov/LVaqgB
I admit I got bored and stopped reading after page 7 or so but from what I read it says the increase in SSD/SSI enrollment comes from the aging workforce/increase in age of retirement (and I can tell you from personal experience, you can eat right, exercise, and your body still starts to fall apart after a certain age, it's inevitable, you can't stop it and it hits women harder and younger than men), the increase in women in the workforce, and rule changes that were enacted in 1980 and 1984 that loosened the definitions of qualifying conditions. IOW, you want to be irked at anyone, get irked at Reagan (the graph mirrors that, with the jumps starting in Bush, Sr.'s administration). That would have been when the oldest baby boomers would be hitting their mid 40s and that's the age the first chronic health conditions love to start cropping up particularly in women.
So here we are 20+ years later and baby boomers are forced to keep working because a lot of them lost their retirement investments during the crash and/or their homes lost resale value and/or companies have discontinued pension plans -- can you imagine it? You get a letter in the mail, you've worked for a company for 30+ years and you had this money, you got these statements, you were counting on it and with the strike of a pen that extra $100k or so you expected to be there when you retired is just gone. My husband was lucky, his company just decided to freeze his but I've heard of them just being ... deleted (I'm sure there's another word for it but I can't think of one right now).
BTW, the report does address the issue of health insurance and how many potential workers may stay on SSD because they need health care benefits and there is a question of how the Affordable Health Care act may impact SSD enrollment.
Old age for a lot of us (in the middle to lower end of middle class especially) is a scary, scary place. It's one of the reasons you go into a fast food joint and often as not the person taking your order has wrinkles and gray hair instead of acne and braces. You can't make over a certain amount of money and continue to receive your cash and medical benefits. Plus, you usually get one meal a day at a fast food place for free or reduced cost and every little bit helps, even if it is only calories and not real food.