That would have been suicide.
The longer he stayed, the more time the Union forces had to regroup and surround him, cutting off his supply lines and preventing him from defending Richmond. The Army Of Northern Virginia were all the effective forces the Confederacy had in the Eastern theater. Lee's job was to fight the Federal army, protect Richmond and threaten Washington DC simultaneously. He could not afford to sit on his ass 100 miles from his closest lines of supply and 200 miles from Richmond.
After the Battle Of Gettysburg - had he been victorious on July 3 - he would have had 45,000-50,000 effectives.
Due to the excellent railroad links near Gettysburg, the Union would have been able to eventually surround him with 150,000-250,000 men depending on how long he decided to sit around.
Had the Union brought up a couple of hundred mortars to target Cemetery Ridge, Lee's command would have been cut to pieces after they inevitably ran out of ammunition and then food.
Lee needed to smash the Union army hard enough to keep it from invading Virginia again for the rest of the season, he needed to stock up on whatever his troops could get their hands on - including horses, mules and beef on the hoof - and then get the heck out of Dodge.