They're also locked/deleted because they almost always degenerate into:
Post 990: Let's get this to 1000
991: Yeah, lets.
992: Fight the Power!
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996: Almost there.
998: Bruce Lee (or Coke)
999: Post 1000!.... edit: Damn, I miscounted
1000: Bruce Lee (or Coke)
1001: Yeah, lets get this 1000 (the 56k non-subscriber finally getting his post in)
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1010: What the hell was this thread about in the first place?