> Evolution has had billions of years to do it, and for whatever reason the conclusion is that either it's not possible, or the cost is not worth it.

For a lot of history dying young was common due to forces that have nothing to do with our own biology. So maybe it's more that environmental conditions did not give any reason for evolution to select for longer life, or select against cells that age. Also it might be that there are trade offs about selecting against cells that age that may have been sucky in the past and not led to fast reproduction, but that those trade offs don't matter now.