I think part of the reason is qualifiers which SGC does not have. If the card has a light pencil mark on the reverse, it won't get above a certain grade regardless of eye appeal. Same thing if the card has light paper loss, glue, pinhole, etc. If a card has any of these defects, it won't reach above a certain grade regardless of eye appeal. Another thing is creasing. I recently purchased a raw 33 Goudey Burleigh Grimes. In the top loader, you cannot see any creases. However, once you take it out of the top loader, you can see the crease. Put it back in, you can have to look really hard at an angle to see the crease again. So with creases like this, even if they only affect eye appeal slightly, can drop the grade significantly.
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