I had some scary tire on my cars before and I can tell you why:
Sometimes one of the tire is bad and the others can still last probably another 5/32. So you get a 5/32 tire to put on the car and in the rear, and wait for all 3 good tires to wear down another 3/32 before you replace them.
Consider only 8/32 of the tires are useful since new at 10/32, this practice saves 3/8 of the total cost which can be easily about $150. If you can get a junk tire to last for low cost, it could be a good investment.
Most drivers do not drive anywhere near the limit of the tires, and would therefore be OK.