The article 'the' is used when talking about monuments, the White House, the Houses of Parliament, so why is it not used when speaking about Big Ben in London?
The difference is in anthropomorphization. Big Ben used to be the name of the huge bell atop St. Stephen's tower, but eventually became the proper name of the whole structure. We only rarely talk about 'the Ted' or 'the Marsha'. Instead, we use the definite article 'the' to convey 'properness' to something that might be otherwise be difficult to distinguish. See: 'the White House' vs. 'a White House'. Big Ben doesn't need that treatment to distinguish itself.