No offense, I don't know anyone who died of leukemia or even had it.
I think you may be experiencing a bit of the "blue car" effect. Of course everyone who went to your school knew someone who died of leukemia. They all knew the same kid.
And once you get to college, I wouldn't be surprised if you had run into a few other people who also knew people who died of leukemia.
But it was not as common an occurrence as you seem to think it was.
It's like 4 per 100k kids per year from ages 0-19, but a pretty high portion of that initial incidence is in elementary school ages -- so 1 per 1500 or so over the elementary to middle school years. And that's just leukemia.
Between school sizes, mixing of schools going to middle school, and auxiliary networks through family and parent networks-- you were pretty dang likely to know someone or know of someone who was affected by childhood cancer. No, it's not a universal experience.
In my extended social circle, I know of 3 cases of childhood cancer that would have had a high fatality rate 2-3 decades ago.
Yea got to agree, no child I knew personally or even knew of in school had cancer. However, two of my close friends in school died of other causes. And a sibling of a friend also died. So I definitely experienced loss.
FWIW, someone I know thought the kid with cancer was getting away with playing hookie, and only discovered what was actually happening years later (when we were randomly catching up). Similarly, through a personal connection to the cancer kid, I learned another kid had survived Leukemia in elementary. And I don't think almost anyone else knew. So it's not really a given that everyone even knows. (At a high school with like 7 or 8 hundred kids.)