I think singing is more natural, you are connected with your instrument which is your voice. I think it comes back more naturally than with a musical instrument.
This is why we many times remember the songs we learned as children. I do remember them, even now, those simple first songs that our family used to sing with us! 😇
This actually makes a lot of sense, never thought of it that way at all.
It is my opinion, but as you sing more than me, you can maybe relate to it better :)
I don't play an instrument so I can't really get a comparison, maybe ill ask around.