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).
First your premise is wrong. Starfleet has multiple missions and purposes, but it's the Enterprise specifically that:

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> Space: the final frontier. These are the voyages of the starship Enterprise. Its five-year mission: to explore strange new worlds
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The Enterprise, both TOS and TNG, has a duty to explore strange new worlds. But that's only part of the mission.

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> to seek out new life and new civilizations, to boldly go where no man has gone before.
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It is being sent to search for new sentient, sapient life. People it can trade and learn from, or from who it should prepare to defend from if they are a threat.

Second, strange has multiple meanings. While the first means different and weird, hard to understand, the second meaning is unknown, foreign, ***alien***. This is in line with the Enterprise mission, seeking new people. It's a first contact mission.

Third, apparently in universe, the Enterprise mission statement comes from Cochrane, from the Enterprise Pilot.

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> On this site, a powerful engine will be built. An engine that will someday help us to travel a hundred times faster than we can today. Imagine it – ***thousands of inhabited planets at our fingertips...*** and we'll be able to explore ***those strange new worlds***, and seek out new life and new civilizations. This engine will let us go boldly... where no man has gone before. *emphasis mine*
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Out of universe, the intention was the same.

Finally, the mission is first contact and planetary charting. Strange planets, ones that don't have any sentient life or ruins detectable, are still noted, scanned, and mapped, **for future research** and follow up by dedicated long term teams. Case in point, Wrath of Khan, a research team returns to the sister planet that Khan was dumped on. Ceti Alpha VI was not expected to have any sentient life. Ceti Alpha V was "barely" Class M as it was too, no sentient life either. Both were cataloged by the Enterprise.

As a parting note, there were only a handful of classification for planets used on screen. Only some of these are considered to have life of any kind. Some classes are Gas Giants and the Demon worlds. Some are life less, atmosphere less rock planets or moons. Class L only had vegetable life. Class h is "generally uninhabitable". Class K needs Terra forming or pressure domes. The Enterprise can scan planets from space for life signs, or signs of civilization like ruins, buildings, man made structures. If a non Class M planet has no signs of life, they catalog it and move on.