Question: Starfleet is supposed to "explore *strange* new worlds and boldly go where no man has gone before", yet most of the time they explore worlds that are more Earth-like than not, and are inhabited by human-like species.

I understand that the out-of-universe reason is that it is difficult to tell a story when the characters are always in space suits, but couldn't the writers have found some other way, rather than skipping all the interesting non-humanoid life forms? I remember the animated series characters using a skin-tight force field (a life-belt?) to explore inhospitable environments.

Or, are there really not that many interesting life forms on other classes of planets?

Answer: 
One reason is philosophical: *Star Trek* was always as much about entertainment and social commentary as about telling science fiction stories -- actually possibly more so. As such, production staff felt that there always needed to be ways to ensure the audience had just enough familiarity with what was going on to feel comfortable and to be able to draw parallels to the real world easily. That, in turn, meant keeping the environments Earthlike-but-weird.

Another reason is pseudoscientific: At the time *Star Trek*, the original series, was being developed, there was a "theory" that Roddenberry felt had some merit (and touts in all the material he used to sell the series to the studios, and in the early writers' bibles) suggesting that the universe was filled with parallel Earths. This is not parallel-universe theory as we now tend to see it played out both in later *Star Trek* and in other stories like *Sliders*. This is the notion that *in our universe* are multiple, separately evolved planets that nevertheless have people and histories remarkably similar to our own. Several stories clearly play on this, most notably "Bread and Circuses" (in which the Roman Empire survived to see technology equivalent to our 20th Century) and "The Omega Glory", which depicts a world in which the Cold War yielded to actual nuclear war. 

And the final reason is budgetary: all those Earth-like worlds meant they could use existing Paramount back-lots, with their pre-built faux-towns and faux-cities that could be redressed to match a variety of needs (or in some cases had been built to mirror a particular time and place for some movie or other and now needed to be used).
I would probably say out of universe they could not make a full episode on planets that had nothing on them. They could only do so much with an empty planet. Whereas class M/Earth like planets they could introduce new species easily. In universe answer I would say that they did but because the vessels in the episodes were not science vessels they were never tasked with such a mission. The Enterprise was the flag ship for the federation, I would probably say combat and diplomatic missions were more important for that vessel. That is my theory.