Why will a miner verify the digital signature, instead of directly adding a transaction to the Merkle tree? I am a complete noob to Bitcoin, and this question might seem obvious to some. But I can't seem to get my head around it. I read how the process of mining works - they select transactions, verify them, create a merkle tree, and then start creating nonces. I want to ask whether a miner can skip the part of verifying a transaction, and just take the transactions as they come, just add them and be done with it? That is, what part of the protocol prevents him from defuncting, in some sense?

Sorry if this is obvious.
If a miner would do so, and the transaction they include is invalid, they would produce a block with invalid transactions in them. As all full nodes on the network (not just miners) verify all transactions in blocks, they would reject this block, and the miner would lose their reward.

By extension, other miners will avoid building on top of such a block, as they too do not want to lose their reward.