A Pronunciation Puzzle

Why do people pronounce “ing” as “in” in such words as “singing”? This is often called “g”-dropping, though there was never any “g” sound there to start with. One answer is that it is a form of laziness or sloppiness, as if it takes more effort to say “ing” than “in”. But there is a decisive objection to this explanation, namely that people don’t “g”-drop in words like “sing” or “king” or “wing”. No one speaks of “singin’ to the kin'” or “takin’ win'”. Everyone who says “in” is perfectly capable of saying “ing” and does it all the time. No one would ever speak of “sin’in'” for “singing”. The puzzle is why this is. It seems to be a rule that the “g” is never dropped in single syllable words but that it often is in multi-syllable words, yet the very same part of speech can be pronounced both with an “ing” or an “in”. Why do people say “walkin'” but not “kin'” or “wingin’ it” but not win’in'” it?