I think it's because, we would live forever, but something goes awry after being mature for so long that our bodies haven't evolved far enough to counteract yet. We solve this by sexually reproducing, which resets the clock on age, so our cell line is still able to live on. We do this instead of resetting our mature bodies because reseting 1 or few cells is easier than resetting/replacing billions that are dysfunctional. So we sorta evolved on the path of least resistance required to live forever, and require further evolution before our mature bodies can do the same, so to speak.

There's no advantage to having a single individual who lives and reproduces forever compared to having his offspring, and his offspring's offspring, etc. reproduce forever.