Cdrzewie

Reason 1: It's hard for the writer. You submit a spec CSI to the Bones people, they might know the show, but they're judging you on concept, pacing, dialogue, general writing skill,etc... But you submit a CSI spec to CSI producers who know every tiny bit of that show inside and out... You're opening yourself up to a whooping. They'll pounce on even the tiniest mistakes. It's like copying a painting, then asking the original artist to judge you. Never gonna win.

Reason 2: Legal... You submit a spec to a show. Now whatever's in your script, they can never do an episode or character beat like that for fear of being sued for theft. And let's face it, there's only so many stories. You send a spec about a dead hooker at the MGM... And they've already got one about a dead hooker at Mandalay ready to shoot -- oops. Now they have to dump the ep they already wrote. It's easier just to have a blanket policy of not reading their own.

Exceptions to the above? Very very few... However -- In the 90s, the various Star Trek shows did actually accept specs of their own. That's how Bryan Fuller started his career.