Rubber "corrosion" is similar to iron rust. More corrosion makes even more corrosion. You need to keep removing any hard or corroded segments to keep the underlying material good.

Storage environment is one of the biggest factors for lifetime of rubber seals. Some will fail via getting harder and others fail by getting softer.

Some of the types of rubber are degrading from the day they are manufactured. It can be trace amount of metal catalysts, free radicals or simply environmental oxygen, moisture, stress.

Failing soft. The way elastomers are made involves a chemical method called polymerisation. It's kind of a special type of zipper - it zips closed to make the elastomer but it can be triggered to unzip back to starting materials. That's where rubbers can fail soft.

Failing hard: A rubber when looked at under a microscope is like lots of little hairs all aligned. When it is sitting there static, some of those hairs will start to cross-link (e.g. you get a knot in your hair or some loose cables in your junk drawer form a knot). This can be residual free radicals (or oxygen induced radicals) that grab hold of the nearest neighbouring strand to make themselves stable. But similar to a relay race, that free radical never goes away, it just gets passed along the chain to the next open spot - then that crosslinks to another strand. Once you have too many knots in your hair or your Christmas lights are tangled up, it turns into a hard non-elastomeric mess.

Using the rubber gasket does involve mechanical/thermal stresses. It will be getting pushed or pulled. Any tiny hard sections will be ground away like a sacrificial layer. It keeps the majority of the seal elastomeric. Unfortunately, leave it static too long and those hard sections will get large.

Other places you see rubber degrading: knife handles getting sticky, plastic toys yellowing with age, shoe soles cracking when not worn, rubber car tyres softening in storage sheds, latex / synthetic clothing turning brittle in the back of your cupboard unworn. A gasket is no different.