Why would miners waste time validating transactions and blocks when they could dedicate their time to mining?
When a miner successfully exploits a block, it transmits the block through the network. As the block begins to spread across the network, each complete node receives a copy of the block and, after validating it, adds the block to the existing chain. Now consider a situation in which the block that is extracted does not comply with the consensus rules. As the complete nodes receive the block, they validate the block to ensure that it meets the validation criteria and, if the block is not valid, the complete nodes will reject the block and will not transmit it further. Therefore, those miners who do not comply with the consensus rules will have their block rejected, and will lose their block reward and the effort they put to undermine the block.
When creating a new block, is it necessary to validate each block? How is it different from validating transactions?
When the complete nodes receive the block, each node performs a series of checks to ensure that the block complies with the consensus rules. Similarly, when a transaction is transmitted, each complete node that receives a transaction will perform checks before including it in the mempool. If the transaction is not valid, the transaction will be rejected and will no longer be transmitted by that node. All transactions included in the block are valid, it is a subset of the verification criteria of the block. You can check the bitcoin wiki page here for a checklist of the transaction and block the validation.