That was the thinking of Longstreet; assume a strong defensive position and wait for a federal attack that was probably doomed to failure given the nature of direct attacks on an entrenched force. That was what happened at Fredericksburg, where all Lee had to do was to sit back and let the AotP decimate itself. Problem with that approach as far as Lee was concerned, was that it would not accomplish his primary goal of dealing the enemy a devastating defeat. Despite the outsized federal casualties at Fredericksburg, Lee was very dissatisfied with the results of that battle because the AotP was able to flee the battlefield intact and replace its losses. At Gettysburg, Lee could have maintained a defensive position on Seminary Ridge (or Cemetary Hill if it had been seized). But Meade did not have to take the bait and attack frontally; with diminishing supplies and tenuous communication lines, Lee could not afford to wait it out for any length of time.
The aggressor is always going to risk annihilation attempting to take an entrenched opponent,. Lee had made the Union forces pay a high price for advancing against his army before and after Gettysburg, so one has to wonder why he choose basically the same tactic McClellan had used against him at Antietam, the parallel between those two battles are obvious.
McClellan out numbered Lee at Antietam, Meade out numbered Lee at Gettysburg, that's the first similarity, Lee was operating outside of his lines of communication, his army had advanced into a Northern state to engage the Union on its own ground.
But there is where the similarities end and roles reversed, instead of establishing a defensive line a d using his reserve to reinforce the line while the Union attacked piecemeal thus squandering their numerical superiority, Lee chose to copy McClellan, and attack an entrenched opponent piecemeal while his opponent could maneuver his reserve to reinforce his lines. .
Even given the chance to split his opponent at the Round Tops on day two, or to attempt a flanking movement, Lee chose to attack straight intobtge enemy in uncoordinated thrust instead of a coordinated effort at both ends of the Union line.
Then there is the issue of the vague and ambiguous orders he issued his subordinates, and the placement of Ewells artillery beyond the point of the fish hook, out of range to support Pickett's attack.
That brings us to the selection of the point of attack order to Pickett, look at the " fish hook " and it becomes obvious to anyone that if a frontal assault is in order, the best spot into carry out an attack is at the bend in the hook, not along the long straight shaft of the hook where the enemy can bring move fire power to bare against troops advancing across wheat fields. At the curve of the hook, only the arms directly to your front can fire on you. At the long shaft enfilade fire will decimate your troops,. And at the bend of the hook, Ewells artillery could hold down union reinforcements from maneuvering from behind the ridge.
So why order Pickett to attack the middle when there are much better positions to hit? This was Lee at his worse, but it wasn't the last time we would witness poor judgement by Lee,