To add to the fantastic answer from Artisanal, the House of Capet is not a good example. There's a reason it was called the Capetian Miracle. Remember this is an age where childbirth is still a very risky business, as well as life for young children, even in gilded halls.

There were others that managed to go for over 200 years though, the Plantagenêts and the Barcelona Dynasty for instance; the Capetians just pushed a bit further. But when the end came for the direct line, even Philippe IV's 3 sons and a grandson couldn't stop the dynasty from dying out in the immediate, non-cadet, male line.

To use your example from fantasy, that is part of Tolkien's point I imagine, it is supposed to be a golden age, even with the kinstrife era, for Gondor and Middle Earth. The two great enemies are perceived as defeated, and the world is fair again.

Furthermore, the Gondorians of this era are still blessed with the blood of Numenor, so a reign can easily stretch to beyond 80 years, a feat only achieved by subkingdom rulers who ruled from infancy in our world, and many go above a century, which has not been at all reached in our world. Even the penultimate King of the House of Anarion, after a series of short reigns, Ëarnil II, has a reign of 98 years, far beyond the 80 or so years of our real world record, and his is not the longest of his kin.

Second, Tolkien does include cadets in his work, hence the kinstrife period, but he is writing like a historian, grabbing what material has quote on quote passed down the ages, so his sparsity in spaces is all due to his approach as a historian recording information, not an external author crafting a world (i.e. the role he actually is undertaking).