Right, see, I said "intelligently." If your DM is treating the dragon as if it is unaware of the existence of crossbows, he is doing it wrong. A smart dragon would fight outdoors only on a windy day (disadvantage for ranged weapon attacks), and it would almost never fight outside the region that counts as its lair. Against ranged attackers, a blue or white dragon would more likely burrow than fly; green or black would prefer to fight deep underwater. They would all do everything possible to keep you from having a clear shot as they let their breath recharge. Also, spellcasting dragons have plenty of ways to stymie ranged attacks.
People seem to enjoy a very restrictive reading of the word "lair," by the way. In my game, if a dragon lives inside a mountain, then that whole mountain is its lair, so lair effects can be used anywhere in, on, or above that mountain. Regional effects extend one mile beyond that. The mere act of getting to the dragon's lair is an arduous process, and a severe drain on resources.
I'm guessing a lot of the so-called "easy dragon fights" went something like:
"Can we fight a dragon?"
"Sure. A dragon swoops down out of the sky. Roll initiative."
Which actually has nothing to do with how dragons should be played.
