Can’t see the wood for the trees (US - forrest)

By focussing on details you miss what is important

e. g. My cousin is doing research into the effect of age, sex, ethnicity, other diseases, lifestyle and obesity on Covid-19. He has so much data he says he it’s very difficult to identify patterns; he just can’t see the wood for the trees.

Its first recorded use is in 1533, by an English priest, Sir Thomas More.
A fun origin might be from the city of Bath, in England. It refers to a concourse of houses designed by architect John Wood. There was a tree directly in front of these houses which grew quite large. So people began to exclaim, "You can't see the Wood for the tree!"