That's because just like in a war, it isn't realistic to expect the frontline soldiers to stay in combat for longer than weeks at a time before they start to exhibit PTSD and their effectiveness begins to drop.
Unless we have large reserves of doctors and nurses to rotate in and out I think we should be very careful not to take them for granted.
328.2 million (Approximate population of the US [1]) * 80% for herd immunity (based on an R0 5.7 [2]) * 1% = 2.6 million people die.
If the fatality rate is 0.5% instead, then 1.3 million people die If the fatality rate is 0.1%, we're still looking at ~260k-ish people.
That's pretty grim.
This situation is still developing so hopefully those numbers get adjusted downwards as later information becomes available.
For reference, ~1.2 million people died to heart attacks and cancer in 2017. [3]
[1] https://www.census.gov/popclock/ [2] https://wwwnc.cdc.gov/eid/article/26/7/20-0282_article [3] https://www.cdc.gov/nchs/fastats/leading-causes-of-death.htm
In the Netherlands a fatality rate of 0.5% would mean it'd take about a year for herd immunity to kick in assuming we managed sustain the peak that occurred about 2 weeks ago (and it's somewhat dubious whether the healthcare system actually can sustain that peak).