They're not supposed to. 5E dragons (and other monsters) should be using "lairs" and allies. A fairly well-organized group of 4-5 can easily trash adult and ancient dragons in a couple rounds without much trouble when they are outside lairs and without allies. In either case, a dragon is an intelligent creature and needs to be played smartly as well. Unfortunately these are built-in assumptions to the dragon stat block, but not explained anywhere in the books.

In my dragon-centric campaign, each attack gains an additional damage die per age category (so adults have 3d6 per claw, ancients have 4d6, bites, tails and wings move up similarly), I didn't modify the breath weapons, I tried 3/4ths max damage instead of average (1/2) and it was just too devastating. Dragons should be scary, regardless of if you find them all on their own or not. Dragons with allies in their lairs should be terrifying.

But if you're feeling that a dragon isn't getting enough damage dice as it gets bigger (which I agree with), just up the die by 1 per age category.