I see your point but even in the best of times many companies still used temp agencies. Back in the go-go late 1990s a buddy of mine worked at Pepsi-Cola. He got the job through Kelly Services ( a temp agency ) and despite the good ecomony, he worked there as a temp for nearly 2 years. Pepsi never did hire him full time claiming that a recession "could start tomorrow and people will stop drinking Pepsi". Strange thing is he was making $11.00 an hour, the full time regular staff was making $11.50plus after so many months Kelly Services had to offer health benefits, 401K and all that and those benefits were almost equal to that at what Pepsi was offering. Neither he or I could figure out the purpose of Pepsi keeping him as a "temp" but I have heard lot of stories of people being in similar situations.
Back to restaurants and temp agencies, considering that a good many of employees in the restaurant & retail business outside of management are part-time who get little or no benefits anyway, for them to go to a company like Manpower, Kelly or whoeverto find employees, to me makes no sense.