Why should a miner include transactions when creating a new block?

Transactions include a fee, which miners are allowed to claim if they include that transaction in their block. While it is legal to not include any transactions, miners who do this will miss out on the fees.

Furthermore, the computational cost needed to verify a transaction is absolutely negligible compares to the work of producing PoW. What is more important is the fact that verifying transactions and building a block on top takes time, time which the miner isn’t mining. For this reason, some miners will briefly work on a temporary empty block just after the predecessor block is found, and will then switch to a full one once the full processing pipelining is through with it.

Lastly, if a miner were to skip transactions for computational reasons, those transactions might (and likely will) still be included by other miners. And when this happens, the miner still has to verify them, in order to build on top of it.