To take a contrived and simplified example, imagine a hypothetical world in which everybody dies on their 90th birthday by default, but a handful of diseases cause 25% of the population to die on their 30th birthday instead. That brings the average life expectancy to 73.75y.
Now imagine a bunch of medical breakthroughs bring the 30-year-old mortality rate down from 25% to 7%. That enormous difference brings the average life expectancy to 85.45y.
That's in increase in life expectancy just shy of 16%, despite the fact that it comes from a 72% reduction in 30-year-old mortality rates.
That's obviously a massive oversimplification, and real distributions look different, but it illustrates the problem with just looking at average life expectancies; it makes it look like everyone's life span has just been scaled up by 16%, which is not the case. If you look at it from the perspective of, "what are the chances my life will be cut short by some preventable disease?", the magnitude of the change is much greater. Some of that change is due to very cheap innovations like bread-mold-as-antibiotic, and some of it is due to very expensive innovations like organ transplants. Either way, I for one am glad those days are over, despite the appealing economic simplicity.
As a specific and personal data point, I have chronic kidney disease, which developed rapidly when I was 25. With pre-WW2 medicine (no transplants, no dialysis), I might have lived to 26, maaaaybe, and even then only if I'd been able to control my blood pressure long enough for renal failure to set in instead of a stroke or heart attack.
Thanks to modern medicine, I'm now nearing 40 with a good prognosis; there's no reason to believe it will impact my life expectancy, and the impact on my quality of life is relatively minimal. Compared to dying in my mid-20s, that's a vast world of difference. I will not willingly go back to pre-WW2 medicine, thank you very much.