They only eat them after the tannins have mostly taken out by rain water. Scientists have discovered squirrels bury different species of acorns for different amounts of time depending on tannin content. There are a few oak species with acorns low in tannin enough for animals to eat them right away, but the vast majority require processing.
The pioneers also leached the tannin acid acid out of acorns and made flour with the results. Also the Native Americans ate acorns, much in the same fashion.
Come to think of it that may be why the Cherokee stole 40 gallons of corn whiskey that my Great-Great-Great-Great-Granddaddy was carrying to town on his mule. Not only that, but too boot they butchered the poor mule, and ate the old man's liver. Do you think that it was a poor acorn year and the Cherokee had no other food sources?
Seriously, I am of the persuasion that the acid content in acorns is intended by Maw Nature as a premergent herbicide whose main purpose besides discouraging things from eating acorns, is killing off competing vegetation so that the oak seedlings can better establish themselves.