Surely everyone who struggles with relative pitch already has in-built relative pitch, so why is it so hard to actually recognise intervals?
What I mean is from a child I (and most other people I imagine) could sing a tune they had just heard. They might sing it in the wrong key but they would still sing it correctly in a key. To do this you surely need to be able to know the intervals in the tune you have just heard and then replay it back with your voice.
However, ask me explicitly to say whether an interval is a half tone or whole tone and I will struggle. How can this be? How can I easy sing a song whose first two notes are a tone apart yet not be able to recognise a tone in isolation?
Another way of phrasing this question is, why can most people learn to identify intervals easily using reference songs for each interval (took me less than 48 hours to do it within 95% accuracy) but yet actually memorising the intervals in isolation takes months (years in my case).
I realise that this question is not just about music theory but neuroscience and that it may not be wholly understood but I am interested to read your views.
Please note that this question is not about any actual instrument. A good answer will also EITHER explain why I appear to have memorised intervals since I can sing tunes well yet not recognise an interval in isolation OR reveal why my being able to sing tunes does not imply that I have internalised intervals. I write this because there are so many answers on the internet which superficially answer this question without getting to the heart of the query