Hashcodes do not have unique inverses. The goal is not just to find any bitstring with a haschcode in the target interval (in which case the solutions would be trivially reusable), but to find such a bitstring that contains information, including a hashcode of the previous block (so you can’t get a head start before that block is computed) and transactions to be recorded (so your block is actually useful to the community).
Hashcodes do not have unique inverses. The goal is not just to find any bitstring with a haschcode in the target interval (in which case the solutions would be trivially reusable), but to find such a bitstring that contains information, including a hashcode of the previous block (so you can’t get a head start before that block is computed) and transactions to be recorded (so your block is actually useful to the community).