They should do this in the medical world too, where surgery tools are sometimes left inside the patient.

Metal has the marvelous quality of showing up well on X-ray, and clothing has the marvelous quality of never getting cancer from X-rays.

Pieces of cotton do not show up on X-ray, and humans do get cancer from too much ionizing radiation.

Of course miscounts happen sometimes, and sometimes both the initial and final counts are one short of the true number, but the vast majority of undetected retained items are sponges made of cotton. Not tools, not even tiny needles. That’s why there is a radiopaque strip embedded in the sponges intended for use during surgery.

It is not perfect, and cannot be.

That’s an expensive solution for a simple problem. They currently just count all supplies in and all supplies out. It works.

If it's that simple, why don't they do it in the fashion industry?

Because there's no practical way to do it on the scale of an entire factory. It works for surgery because you only do a single surgery in a single room on a single person with a countable number of supplies.

Factories don't work that way.

Clothing doesn't get cancer. Also a lot of what can get left in a patient doesn't show up super well on x-rays, so more general solutions like counting in and out are preferable.

Absolutely, this is more common than many think

Also the food industry, especially meat where you want to avoid bones in unexpected places.