Because evolving doesn't have a point other than total population alive evolving as balancing forward uniquely here, now. There was never an actual tomorrow on any calendar recording human behavior every generation gaps lived so far.
Calendars are just journals of how humans behaved daily with the population never same ancestors twice living in series parallel sequence of inception to conception, dead to decomposed each conception forward left alive now until each great great grandchild replacing its previous 4 generation gaps fails to add another generation of great great grandchildren added one at a time today.
Simple compounding chromosomes and rule of 72 with the touch of thermodynamic principles as salt and pepper.