Hi there! I'm afraid there's no other explanation except "This is the way things are".

The ending is pronounced identically for both singer and longer.

Thank you both Chris and June!

It usually depends on the entomology of the word. English is made up from several languages and words that may seem like they should have a similar pattern but don't (Foot/Feet, Boot/boots) often have a different root language so their rules are different. Spoken language is similar, especially if we consider the dialects of not only the original language but how different modern language is to say Middle English. It could be at one time the they were the same, but you would have to look at the history of the words to know, and I'm not that curious ;)

As should be clear from the range of different answers, it really depends on your local accent. Personally (I'm from the North of England) I pronounce them with the 'ng' exactly the same, but even then on reflection I've realised that's probably not even consistent all of the time. I'm not even sure how dictionaries decide on the IPA, given that there's no central authority on pronunciation of British or American English. TLDR; Don't worry about it. :-)