Because it's 36 years old and that is what happens when it is in circulation and a bit of Environmental Damage Not a Mint Error and only worth 1 Cent
352sdeer, posted: "That planchet might have been a touch improperly annealed. Kinda looks porous." I find it's best not to over think things. Often the simplest answer is the correct answer: paddyman98, posted: "...a bit of Environmental Damage."
I keep almost everything I find so I have to come up with categories to file them within. Because of this I have a habit of trying to focus an individual coin into a grouping regardless of its value or numismatic importance. I’d file this coin in the planchet errors under Porous planchets. Just me though every one collects a little different. The average collector probably shouldn’t concern oneself with such minor stuff I realize and don’t recommend it to most. But I do love my Lincoln’s so. Reed.
Believe it or not, You are way above an "average" collector. The "average" collector doesn't know what a planchet error is.