Why you need to rehash each block after a changed block when changing data in a blockchain? Maybe my question is stupid, but, why you need to rehash each block after a changed block when changing data in a blockchain? 

Each block contains a hash of its data and hash of previous block. If I change the data in block № 9000, then block № 9001 will refer to a nonexistent block. The Chain will be broken. Every article that I read says something like:


  To make this work, you will need to rebuild and rehash each block
  following the tampered block, replacing the contents of the
  previous-block-hash pointers.


But, if block contains only hash of previous block, why can't I just change the previous' block hash value in block № 9001 if I change data in block № 9000? Wouldn't that restore the chain?
The blockhash is a hash that includes the previousblockhash as input.

Suppose you are trying to change the data in block 9000. You modify it the way you like and then successfully mine the modified block (find a hash that meets the difficulty threshold at that height). You now have a block 9000 with a new hash, so block 9001 has the wrong value in previousblockhash.

Your theory is that you can simply update the previousblockhash in block 9001, but since that field is part of the input to the hash of block 9001 modifying it changes the blockhash. Now you need to successfully mine block 9001 with the modification and once you do, block 9002 will need to be modified to have a new previousblockhash referring to the new blockhash for block 9001 and then it will need to be mined as well, and so-on, and so-on.