When you speak ‘Goodnight’ the last letter of the word ‘Good’ i.e. ‘d’ is spoken using the tongue and palate which is similar to the first letter of the word ‘Night’ i.e. ‘n’ which is spoken using tongue and the inner side of the upper teeth.
When you speak ‘Goodnight’ you don’t move your tongue from the palate to the inner side of the upper teeth while changing the words and say it like a single word. This might be one of the reasons why ‘Goodnight’ is written as a single word.
I copied and pasted. :/