Why Do Miners Accept a New Block Even If They Haven’t Finished Their Own Work?

Miners accept a new block because the network operates on the "longest chain rule." Once a new block is broadcast and verified, it extends the longest chain, and any work a miner was doing on the previous block becomes obsolete (stale). Continuing to mine on a stale block is a waste of resources.

Therefore, miners immediately stop their current work, update their block header with the new block's hash, and begin mining on top of the newly accepted block to maximize their chances of earning the next reward.