I think Lee had very long supply lines which were vunerable and he didn’t have time on his side. Thus going offensive was a good option before the Yanks started to hit his long supply tail.
OTOH, he should have know that before he wandered in Pennsylvania.
Everytime Lee went into Yankee territory he got hit hard. Maybe offensive action in enemy territory was his weak point?
I think Time was always THE biggest enemy of the Confederacy. The confederates kept pummeling the Union in the East, but they just wouldn't quit; meanwhile, the north continued churning out soldiers and weapons in numbers the south could never match. I think I once heard (so I could be completely wrong) that Meade had two complete corps sitting around at Gettysburg that were never engaged - which makes Lincoln's fury at him for not pursuing Lee immediately after the battle much more understandable.
Grant’s offensive in TN removed that state from the southern economy. As such the southern soldier was unable to get much pork, TN’s major contribution to the southern economy.
In response to Grant’s successful offensive there, the pretended confederate government tried to get farmers to grow poultry for their own needs, and for supplying southern armies. Grant’s and Sherman’s campaigns at Vicksburg and the March to the Sea could not have taken place without that change to the southern economy.
Lee was trying to take some pressure off the remaining southern farmers such as those in the Shenandoah. He was partially successful, as the losses he suffered didn’t show up at the dinner table ever again.