What prevents this is the subsequent blocks.

If you mine a replacement block for block #199999, you change its hash.  Because that hash is stored in block #200000, you'll change the content of that block and have to remine it.  Then you'll have to do the next block and so on, all the way to the end of the chain.  As of this writing, that's another 31375 blocks.  Nontrivial no matter how much brute force you throw at it!

Solving a block doesn't mean brute forcing for a specific hash.  It means brute forcing for a hash in a particular range; many valid hashes would be possible, so it's a much easier problem than looking for a specific hash.  So you can't simply try to solve a modified block #199999 for the same hash as the real #199999; that would be a much more difficult problem.