I have come across in Crash Course World History. It is at around 43 second. Here is the context:
When we think of ancient civilizations, we think of Egypt. There a few reasons of this, like the fact that the pyramids are the last man standing among the seven ancient wonders of the world.
Literally, the phrase "last man standing" refers to the sole survivor of a battle, when everybody else has fallen.
Figuratively, the phrase is commonly used to describe anything or anyone that has outlived, or outlasted other similar things. It is commonly used in "survival" video games to determine that there will be one final winner. As in your question it is sometimes also used to describe something inanimate - for example you might read that Blockbuster Video is "the last man standing" because all other video rental chains have closed down.
I think it means that pyramids are the only wonders still found today! Rest all are vanished into the thin air.
And, because of that, even today, when we think of ancient civilization, we think of Egypt because the evidence is right there in front of our eyes.
Note that when the video has pictures of rest of the wonders, all start getting crossed which means they don't exist anymore except for the pyramids. This backs my stance that last man standing means the one who survived the battle of existence.
The video is referencing the Seven Wonders of the Ancient World, which were "must see" tourist destinations first written about by Philo of Byzantium in 225 BC in his travel guide "On the Seven Wonders".
They included:
Due to the passage of over 2000 years, 6 of those 7 no longer exist. The only one you can still visit today is Egypt's Great Pyramid.
As mentioned in Astralbee's answer, the only remaining member of a set is sometimes referred to as the "last man standing". It is a phrase that invokes a battle or some sort of contest, but is understood in a wider context as well.
