Etymology of the phrase "cannot see the forest for the trees"
Solution 1:
In English, this saying goes back at least to the 16th century:
1546 J. HEYWOOD Prov. II. iv. (1867) 51 Plentie is nodeintie, ye see not your owne ease. I see, ye can not see the wood for trees. —Oxford English Dictionary
This is for in the sense of “on account of”, “because of”, which goes back as far as Old English. (“for”, Online Etymology Dictionary)
You can read the saying as:
Cannot see the forest because of the trees.