Because in the long run, energy costs will account for the majority of mining costs, and only when the benefits are greater than the costs will the miners work and the network will be safe. As the block reward is halved, if the price remains unchanged, the miners' income will also be halved. At this time, some mining machines may have to shut down. Bitcoin is unlikely to experience a price surge after every halving. Once the block reward reaches a certain level, it may pose a threat to the security of the entire network.