Part of it was that because so many more were infectious disease -- and violence, and accidents -- they were more spread out over the expected lifespan. Death could hit at any time. They had art showing the Dance of Death, in which a skeletonal figure would dance with men and women of all sorts of conditions and ages
It was also the way people died -- at home, in their own beds. L. M. Montgomery had a novel in which you acknowledged that death was near by shifting the dying person into a specified room in the house. Then the body would be prepared by the family or friend. One of them would carpenter the coffin. They'd bring the coffin to whatever funeral there was, and they'd dig the grave.
It was a lot harder to pretend that death was somehow a regrettable accident instead of inevitable.
True, some writers go overboard the other way about how they regarded death. The Stoic whose response to his only son's death was, "I was aware I had begotten a mortal" was indulging in a philosophical position, not a standard reply. There is a lot more reaction to death. Even of the very young -- the author of Pearl found the death of a two-year-old daughter suitable for a lengthy elegy. Though, to be sure, in ancien regime France, for instance, where small children were parked out in the country for several years, it was not the custom for mourning to be worn for children under five years. Then I ran across that tidbit in the course of reading how some people had donned mourning nonetheless.