When stamp grading first started, stamps were enclosed in plastic, just like what has been done to coins for many years. That service is still available. However, most graded stamps today are not enclosed in plastic. It’s kind of hard to mount a small slab of plastic in an album. Therefore, collectors have not widely accepted sealed stamps.
There will come a day when encasing stamps in plastic may be necessary to protect them.
The first official postage stamp was the 1 penny black from Great Britain issued in 1840. That stamp is almost 175 years old. The first US stamp was issued in 1847. Older stamps like these, especially mint copies, are becoming brittle with age. The more they are handled, the more likely they are to become damaged.
When the first stamps were printed, no one thought about collectors saving these stamps for centuries on end. Therefore, the paper has high acid content in some cases. And the gum was made from various chemicals that made the stamp stick to the paper. “Never hinged” wasn’t even a term in the mid-1800s. Left alone, these stamps will someday disintegrate