If they could, miners probably would put all the pending transactions into a block. That's why the block size limit exists; every other node in the network has

If they could, miners probably would put all the pending transactions into a block. That's why the block size limit exists; every other node in the network has to process all those transactions and they're not paid for it in any way, so without the size limit miners would be likely to act in a way that hurts the rest of the network.