it's related to the blockchain technology. if you modify one block, then this block won't be accepted by others and discarded at last. you're definitely not the first who thought about this, the designer had already figured the solution 
It's not about modifying the block. The sole reason why this doesn't work is due to the fact that the Bitcoin nodes enforces the rules individually. Any deviation from the rules will not be accepted, even if the block is valid for the most part. Modifying any block would make it invalid regardless.