old world

Because most of the people who originally came to the "New World" which was America, were originally from Europe...therefore, Europe was the "Old World" and America was the "New World".

there's so many old buildings and architectures.

Is it called that? It may be the first time i have ever heard this. But the reason can be easily guessed out. But at the meaning of the word OLD, the whole Eurasia and the Africa can be called the old world, and the America and the Oceania can be called the new world, especially hundreds of years ago.