That would violate the progress-free property of mining, which is very desirable.
If miners have to construct multiple PoWs per block, that means they (private) accumulate points, and the first one to reach enough points wins the block. In the extreme, with a long enough chain, this will imply that the fastest miner always wins, which is very anti-competitive. Even with just a few PoWs per block, there is a strong benefit to larger miners. In contrast, a single PoW per block functions as a lottery, and miner win probabilities scale proportional to hashrate, and thus to investment.
