Most likely because the founding fathers didn't expect the justices to be split on party lines. They were intended to be impartial judges ruling on a limited set of federal laws.
https://clickamericana.com/topics/health-medicine/us-life-ex... https://www.everydayhealth.com/senior-health/what-life-expec...
Those probably aren't the best places for me to get numbers, but I think they're good enough for a rough example of what I mean.
I used the historical list of US Supreme Court justices [0] and averaged the lifespan of the first 20 justices. The average was 67 years. Maximum was 91 [1].
[0] https://en.wikipedia.org/wiki/List_of_justices_of_the_Suprem...
This is false. The statistic doesn't imply this. Longevity has not increased significantly, it's that life expectancy from birth has increased because childhood illness became less of a concern. Once at adult age people generally still lived long, ignoring accidents which really wouldn't be a concern for the type of people who would be justices.
It is not like people dropped dead at 50. People who survived usually hit their 70s even when the expectancy was in 30s and 40s. The average is low because lot of kids and people died young.
The proper metric here would be life expectancy of active lawyers/judges in their 30s and 40s . They have double advantage of both generally surviving all the diseases till then and wealthy and educated enough to afford medical care available.
Even if the idea was that people may not survive long, it is a unpredictable and inconsistent way to set policy.