Why does toasting bread make it hard?
Two reasons; first is simple dehydration. Water makes things pliable, so less water means a tougher substance. That’s also why toast cools so much more quickly than warm bread.
The second is much more complex, almost beyond the scope of ELI5. It involves the breakdown of relatively complex and pliable long chain carbohydrates, bound together with springy gluten. High heat shatters these fibrous molecules into simpler sugars; and these short chain sugars imbued with energy from the toasting process form orderly lattices with one another.
In effect it’s extremely similar to making hard candy: ‘crystallization’.
This is the reason why overtoasting leads to blackened, crumbly bread. Eventually you chop the components too short and they become nearly pure carbon, forming tiny fused clumps instead of orderly planes of tasty bread.
When you toast bread, you are dehydrating the exterior, and by removing moisture you hard the bred harder (just like when it goes stale). Unlike when it goes stale, the heat is causing something called the maillard reaction to happen, which is the chemical reaction in food that makes crust of bread, the hard exterior bits on steak, etc. During the maillard reaction, sugars and proteins both caramalize and produce complex molecules that weren’t in the food in the first place, and give the depth of flavor we love about the exteriors of cooked foods!