It’s because layering doesn’t really gain you anything so it only has downsides. It’s important to differentiate encryption and hashing from here on since the dangers are different.

With hashing, layering different hashing algorithms can lead to increased collision chance and if done wrong a reduced entropy (for instance hashing a 256 bit hash with a 16 bit hashing algorithm). Done correctly it’s probably fine and in fact rehashing a hash with the same algorithm is standard practice, but care should be taken.

With encryption things get much worse. When layering encryption algorithms a flaw in one can severely compromise them all. Presumably you’re using the same secret across them all. If the attacker has a known piece of input or can potentially control the input a variety of potential attack vectors open up. If there’s a flaw in one of the algorithms used that can make the process of extracting the encryption key much easier. Often times the key is more valuable than any single piece of input because keys are often shared across many encrypted files or data streams.