Two different types of gum
We have two different types of gum in our oral mucosa. keratinised epithelium (or keratinized epithelium for you Americans) and non-keratinised epithelium. Keratinised epithelium is essentially toughened gum. It is skin which has many rough long strands of keratin (the stuff which makes up your hair and nails) interwoven within in the skin. Non-keratinised epithelium is simply normal gum which is normal and does not have the extra reinforcement.
Take a quick look at this picture to see the two separate layers (the pink gums near the teeth is the keratinised epithelium).
Keep in mind - that our bodies are if anything SUPER efficient. There is very little waste, and skin - having a turn over rate of around 7 or so days means that every 7 days or so, the old skin (or gum) is shed, and new reinforced gums are produced. This toughened gum (keratinised epithelium) is expensive upkeep!
So why does our body "waste" extra precious resources to toughen up these parts of the mouth (a thin layer around our teeth, and our palate).
Well, when we chew - especially hard foods back when we didn't used to cook our food, our teeth would cut what we're eating, and the hard food scraps would tear into our gums! This toughened gums are there for necessity!
When our toughened gums thin out, or disappear we're in trouble! Even ordinary actions such as tooth brushing or eating would eventually exacerbate and cause gum recession