It is all in the application, mechanical fasteners (screw, nuts, and bolts) are meant to hold things in place but also allow removal. Glue is for a permanent bond that is not meant to be removed.
I have seen both fail.
Fasteners usually fail from under or over tightening or using the wrong type. If you are constantly loosening and tightening a screw, especially into wood, the threads will eventually just make drill a hole. If bolts and nuts are always coming loose then you may need to use a lock washer or torque the bolt tighter. If bolt or screw is breaking off you may need to lessen the torque or use a larger bolt or screw.

Glue usually fails due to temperature or application, some glues don't work very well in certain climates (cold, hot, humid) also many times they will not stick to certain surface. Glues usually have detailed instruction on how to use for best results, most have a setup time, or time until the glue hardens, and a maximum and minimum temperature that the glue can be used in.

The truth is most of the time people, including the manufactures of many house hold products, just think a screw is a screw and bolt is a bolt, if you are not using the right one it will inevitably fail, but if you never plan on removing it glue might be your best bet.

Most items do not work correctly or fall apart due to faulty engineering or manufacturing. Correct tolerances are built in to a design of a product (in theory) and the manufacturer is contracted to produce the product to the best of their ability. Well this does not always happen so the consumer is at the bottom of the food chain when it comes to a manufactured product and its reliability.

If you are suggesting that screws, nuts, bolts and other fasteners fail more often than glue, then I couldn't disagree more. I'm an engineer by trade, and have found that the only place glue is better than a mechanical fastener is with wood products (paper, lumber, etc) and with welding for metals and plastics. In every other case a mechanical fastener is far superior in terms of reliability and longevity.

Most often the reason for failure is the improper application of a given fastener. The proper fastener must be used and the directions followed ie. clean surfaces, correct torque, correct size for the application. If a nut and bolt are used and the items fastened are subject to vibration a lock washer or lock nut must be used. Screws must be the correct size and have proper thread style for the material fastened. Fasteners are engineered for a specific job and should not be interchanged.