What is the incentive for including transactions in a block?
Three reasons:


Leaving the block empty doesn't save any time. They still hash however many times a second they hash.
Transaction fees make up a significant fraction of the mining reward today. Such a miner would be foolish.
Miners have invested in expensive hardware that is valuable only if bitcoins remain valuable. If too many miners mine empty blocks, confirmation times will go up and the usefulness of the network will be diminshed.