Partly because Hitler ordered that specific cultural sites shouldn't be targeted. Partly because a lot of buildings in London actually were destroyed then rebuilt close enough to what they were that most people can't tell the difference. For example the whole south side of Fleet Street was reduced to rubble near Temple Bar but you wouldn't know it now.
Hitler might have made that order, but that doesn't mean that WWII bombers were actually able to bomb with such precision that they could deliberately avoid hitting specific buildings. Especially since the bombings happened at night! The blitz was "strategic bombing" which basically means a large area is bombed in order to destroy and and kill as much as possible - not target specific buildings.
