In TOS, we're introduced to something called Hodgkin's Law of Parallel Planet Development, which justifies why so many planets have Earth-like societies.

While introduced in "Bread and Circuses," it's certainly not the first time we see an astonishingly Earth-like planet that's excused by happenstance; we also get this, one of the (perhaps unintentionally) funniest lines from TOS, after the crew comes across the Earth clone in "Miri":

Captain's Log, stardate 2713.5. In the distant reaches of our galaxy, we have made an astonishing discovery. Earth type radio signals coming from a planet which apparently is an exact duplicate of the Earth. It seems impossible, but there it is.

Then later in TNG we learn that the primordial soups of untold numbers of worlds in the Milky Way were seeded by Ancient Humanoids intent on driving their evolution towards specific forms.

While none of this directly explains why the geography of so many planets is similar, it certainly points to a trend in the Star Trek universe that Earth is "normal," read: not exceptional. Thus, after a while, it ceases to be surprising that so many planets are Earth-like, in the same way it would be unsurprising to examine all the bodies of water on Earth and determine that they all contain hydrogen and oxygen.

Essentially, in the Star Trek universe, Earth-like planets are ostensibly one of the fundamental building blocks. For a less hand-wavy theory, though, I think selection bias could be responsible (as already mentioned in other comments here).