Understanding Bitcoin - Usage of Merkle Trees

I'm currently learning about the Bitcoin blockchain.
I've come across the concept of Merkle trees in the context of Bitcoin. I understand that Merkle trees are used to verify the integrity of the data, but I'm struggling to understand why this is necessary when we can query the database directly. Can anyone shed some light on the practical use of Merkle trees in the Bitcoin blockchain? Why can't we simply query the database for the transaction data?