So the real reason that's not being explained is that our enamel is softer and weaker after a night's sleep.

So, in your mouth, there is bacteria. Even brushing as well as you can, there's a microbiome that's ever-present in your mouth (this is why traditional mouthwashes are so bad - they unbalance ALL bacteria in your mouth, even the good stuff).

This bacteria stays pretty much in balance in a well-taken care-of mouth, as long as there is saliva present. Saliva washes away food particles that bad bacteria feast on, boosting their numbers and creating bad odor. But, more importantly, when bacteria feast on sugar, it turns to acid.

This acid softens our teeth, which is why those with chronic dry-mouth will often suffer cavities more frequently.

When we sleep, saliva production pretty much halts. That's why you wake up with a cottony mouth. If you eat right after waking, you're basically converting any sugar you eat directly into acid that goes to work at your teeth.

By brushing prior to eating, you "restore" your mouth's balance, stimulate saliva, and are ready to eat.