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They've thought about it. Such questions are kind of funny, because they take for granted that the scientists' standards are akin to the pop science stuff that everybody else writes. They aren't, and that is what makes good straw man arguments.

I submitted yesterday -with little success- a nice talk of Jonathan Lunine (Cornell) at the Harvard-Smithsonian Center for Astrophysics, here it is again: https://www.youtube.com/watch?v=m_mb59aQblY . This, albeit introductory, is what scientist do.

If you find a planet with an atmosphere rich in free oxygen, you can bet it's life that has put it there, because we have a pretty good idea about the abundance of the elements and other scenarios are unlikely. At the beginning of the Archean, there was no O2 in our atmosphere. All there is now comes from water, CO2 and photosynthesis. As scientists are usually serious about their jobs, abiogenesis models take into account the more likely primeval atmosphere and conditions: https://en.wikipedia.org/wiki/Abiogenesis

What is exciting about the search of life in Europa, Enceladus, maybe Ganymede and who knows Triton (apparently there's no will to go there ever again), is that we have very good reasons to assume that at some depth under the ice crust there are hydrothermal vents like those on Earth. And maybe chemosynthesis is enough to get things started.

Personally I find that there are more reasons to be excited about Enceladus than about... let's say Mars.

>How confident are scientists that life on Earth began in the conditions of present-day Earth?

My understanding is that life did not begin in conditions anything like present day Earth. In particular the oxygen you are currently consuming was produced by living things (https://en.wikipedia.org/wiki/Cyanobacteria). Prior to there being oxygen producing organisms there was very little oxygen available in the atmosphere.

So, perhaps they are looking for an earth-like atmosphere on the assumption that a similarly oxygen rich atmosphere would mean that there are oxygen producing organisms present.

We know that current conditions support life. We don't know which of Earth's past conditions necessarily lead to the creation of life.

Gotta play the odds.

>All the talk about looking for earth-like planets to find extraterrestrial life could be a big waste of time if we eventually learn that present-day conditions on Earth are not conducive to creating the fundamental building blocks for life

The only way we can do this is by exploring other worlds. We can only infer what happened in our past, but if we see life elsewhere we can assume some general set of conditions that caused it. If we look and don't find it, then we must assume life on earth is a fluke.