Did God tell us when, why, and how life spans would be drastically reduced? What? NO??
Maybe He will 'splain Himself to you eventually. For now you just have to either accept God's Plan, or kvetch on why He didn't first deliver that Plan and check in with Mr. Trebb.
"Then the LORD said, "My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years."
I realize that many say this has nothing to do with limiting our lifespans, but before (and for a little while after) this, 300/400/500 years was not uncommon and some lived much longer. After Moses died, it seems that lifespans kept decreasing.
Mock and ye will be mocked.