The human lifespan is so short because our bodies are not meant to live any longer than 70 years to 122 years.
122 years old is about the maximum lifespan that a person can live too and even then living to 122 is both luck and staying healthy and active.
Our body parts and organs such as our heart, liver, kidneys etc cannot work forever and so eventually they stop working and we all die.
However when the human lifespan is compared to other animals lifespans such as a dogs lifespan, cats lifespan etc then our lifespan as a human is not all that short.
Humans can live from 70 years to as long as 100 years and sometimes longer.
While dogs and cats typically live from 8 years to 12 years and sometimes 18 years to 20 years.
But when compared to a human vs a shark some sharks can live up to 500 years so then humans lifespans could be considered short when compared to sharks lifespan.