Why are Ethereum blocks not filled to the 30M gas limit every time?

Outside of case where network activity is so low that there is not enough transactions in the mempool to fill a block to the gas limit. Why does this happen? The most gas is used in the block the more money the proposer makes right?What I am asking, is as block builder when I included the most profitable MEV bundles, the transactions with the highest priority fee, and I am still under the block gas limit... Why not add any random transaction until I reach the limit?