In J.R.R. Tolkien's legendarium dragons have soft unarmored bellies and so some of the largest and most fearsom dragons were killed by puny men stabbing them in the stomach area or shooting them in the breast area.
That is how some of the most famous dragon slayings in the legendarium happened.
And I have reason to believe that those dragons were so large that when they stood up straight on their legs their bellies would have seen so high above the ground that someone would have had to jab straight upwards with a long spear to reach them.
And maybe Tolkien's dragons always crawled along the ground in battle, with no space between the ground and their stomachs for any enemy to stab them. So the only places where an enemy could stab or hack would be the sides and the tops of the dragons, protected by armored scales. So maybe Eru could make Morgoth think the dragons didn't need any armor on their unndersides. But of course that is inconsistent with how Azaghal stabbed Glaurung at the Battle of Tears Unnumbered, so apparently Tolkien's dragons didn't aways press their undersides against the ground for protection in battle.
So it always seemed like a major design flaw that Tolkien's dragons had such vulnerable unarmored undersides. How could Morboth have been stupid enough to design dragons with such fatal flaws? And of course the answer could be that Eru made Morgoth fail to see the fatal design flaw, because Eru wanted to make it possible for great heroes to slay dragons.
But if a writer doesn't specifically want to make it possible for a great hero to slay a dragon in his story, because he is telling a diffrent type of story, then they have no need to give their dragons vulnerable, unprotected undersides (or unprotected sides, tops, tiny spots right above vital organs, etc.).
Possibly when a dragon attacks an army, or an army attacks a dragon, in a story, the dragon can walk over the army formations, with their well armoured underside 20, 30, or more feet high, and out of reach of their enemeies, and then suddendly fold their legs and come slamming down on the soldiers below, crushing tens or hundreds to death, with vertical spears and swords snapping like toothpicks. That would crush the hopes that any character might have of being able to stab the dragon in the soft underbelly. And also crush the character if they were under the dragon at the time.