Because it would cost more, and it probably wasn't felt it was necessary. I have to agree with them for the most part--there are a few scenes that called for re-comping, but the films generally held up well. There's only a few instances where matte lines are obvious (many of which were fixed), and garbage mattes only show up on home video.

Of course, I'm speaking of the 1997 SE. The 2004 SE cleaned up more mattes and probably re-comped some more stuff, but the blacks were crushed so much that if you can still see garbage mattes your brightness setting must be way off.