There's just one problem: people's lives get ruined by the lack of security and benefits.
Honestly, it cuts both ways.
When you are a "temp" you're still often a full-time benefited employee of the contracting firm, or your paid so well, you can source your own benefits. At least that's the case with situation's like we're discussing here, Boeing, which were certainly engineers, accountants, and IT folk. This can create enhanced job security, and actually has worked in my favor in the past.
If the end-employer is making cuts or is going out of business, the contracting firm may have other work lined up for you. Or if you are valuable enough, they have "bench pay" to keep you in their employ until they find you another job site. The direct hire employees who were supposedly "better off" are thrown back into the job market to fend for themselves.
I've been at more than one business as a contractor where the entire IT department was getting cut, usually for a offsite data center (still within the company, but located out of state) and all the direct hire employees who enjoyed "stability" and always looked at contractors with a mild case of disdain, or perhaps pity had their eyes opened. Most all of us either had bench pay, or other jobs lined up with our firms already. The few who didn't were niche consultants who were pulling down three figures an hour and didn't care if they were out of work for a few months. To them it was a vacation.