In fact, it's not even on the real spot.
|View of Walden Pond|
from Thoreau's actual cabin
The real spot wasn't re-discovered until the 1940s or 1950s.
By that time any remnants of the original cabin were long gone. Or were they? A local Thoreau buff found it by way of some amateur archaeology work: He located the chimney stones and even more importantly, the actual hearth of the fire place. It's located about a third of the way around the pond from where the replica now stands.
Why did Thoreau chose Walden Pond?
It was the last remaining spot of forested land as far as the eye could see (from the bell tower in Concord). The rest was all cleared (to supply timber) and for farmland. Ironically, there's more woods in New England today than in 1845. The reason for that is that wood is no longer used as the predominant form of energy for heating homes.
And it's a good thing, too: Today's homes are much bigger than Thoreau's simple cabin.