“Why” implies an underlying ontology. Maybe there is something underneath it but it’s as far as it goes down as far as we currently know. If we don’t at least tentatively accept that our current most fundamental theories are the fundamental ontology of nature, at least as far as we currently know, then we can never believe anything about nature at all, because it would be an infinite regress. Every time we discover a new theory we can ask “well why that?” and so it would be impossible to actually believe anything about nature.

What is the distinction you are making between knowing the math and understanding it?