Anyone knows theory behind this idiom?
Re: Rome was not built in a day
It was the centre of Roman empire with one of the most brilliant architecture, Vatican is also part of rome, and all europena languages are written in roman script.
Re: Rome was not built in a day
Yes all the big civilisation were made over the years. Why they empasised about making of Rome over the period? Was there any myth that said Rome was made in a day 
Re: Rome was not built in a day
There wasn’t a myth , but rather the idiom is a lesson that the greatest things takes time to achieve.
Re: Rome was not built in a day
The ancient biographer Suetonius told us that Augustus:* found Rome a city of brick, and left it a city of marble*
A marble city that befitted it’s place as capital of one of the greatest empires ever known and in the words of Edgar Allen Poe, ‘the grandeur that was Rome‘.
Obviously this grandeur *took some time to build, from the establishment of the city by Romulus to the later emperors, we’re looking at a span of 1200 years.
When you say that ‘Rome wasn’t built in a day*‘ you are pointing out that you shouldn’t expect great things to be done quickly.
Re: Rome was not built in a day
That means Rome was built in 1200 years. By this formula, we should not be sad about Pakistan’s progress in 65 years.