Most modern cryptographic hash functions use some form of compression function combined with a construction such as the Merkle-Damgård (MD5, SHA1, SHA2, etc), the Sponge construction (with Keccak as a notable example) and variants of these constructions in order to allow messages of indefinite length to be processed. It also happens that in order to exploit parallelism some hash functions have modes which make use of Merkle trees, such as the tree-hashing mode of BLAKE2 and ParallelHash (not a Merkle tree per-se but still).
To my knowledge there are multiple Merkle tree-like structures that provably reduce the security of the whole construction to the one of the underlying primitive (for example the one described in the Sakura paper and the one described in the Certificate Transparency RFC).
If I am not mistaken MD6 tried to use Merkle trees directly, however I am not aware of any other cryptographic hash function trying such a thing. Is there any reason why more cryptographic hash functions don't use a Merkle tree-like construction directly instead of just adding them as a way to allow parallelism to existing hash functions on top of the Merkle-Damgård or the Sponge construction?