(Comparison with indent-based program programming languages like Python and Haskell is not completely appropriate, because one typically avoids too many nesting levels when writing code. But in a large YAML file you might just need to go deep.)
If it were JSON, I could navigate within a subtree by selecting & searching the text between the `{}` delimiters. But instead, I have to gently scroll downwards, keeping a careful eye on the indentation, to see whether I've moved into the adjacent subtree.
An example of my common use case is digging through a Bosh manifest file for a resource with a particular name, then finding, say, the VM type associated with that resource. The name and vm_type fields may be separated by hundreds of lines (it's a machine-generated file that does not retain element order), but they're siblings of each other.
In vim, I could land on the first { and then hit:
v% (extend selection to matching delimiter)
/\%Vvmtype: (to search for "vmtype:" within the selection).
It's absolutely correct that this is a tooling problem. You could imagine a YML plugin for IntelliJ or vim smart enough to offer a motion for "select all the children of this block," but AFAICT no such tool exists.
I'm sure that some people will scoff and point to anchors as exactly the kind of complexity a configuration format shouldn't have. But I've personally found them extremely useful and straightforward.