Why could organisms change in a way similar to a mountain even though organisms don't live as

long as mountains last?

Organisms change gradually through evolution on a timescale similar to the formation or "evolution" of mountains.

Explanation:

Organisms may have lifespans of irrelevant length compared to a mountain, but just as mountains are slowly formed over eons, organisms gradually change and diversify through evolution.

Organisms live and die to weed out inefficiencies in their species-over time, the adaptations formed by evolution drastically change the organism, mirroring the gradual change of flat land on a tectonic plate forming a sky-high mountain.