I am not aware that scientists are "quick to dismiss" life on other planets. They generally assume it's a statistical inevitability; the only question is what the actual odds are. In particular, "lower" life forms should evolve wherever the conditions are right; "higher" forms might be less likely and in any event take longer to evolve. We might have to go hundreds of light years to find life, organic or otherwise.

In the irony department: there is an old sci fi short story, "Made of Meat" that discusses alien sentient machines in a universe that turns out to be mostly sentient machines; they encounter humanity and can't believe that a sentient being can be made entirely of nothing but meat. You can Google it and read it in about 5 minutes. Sort of looks at it from the opposite direction.

There are also a couple of Star Trek episodes that deal interestingly with the question, what if we don't recognize alien life when we see it. The Devil in the Dark (original series) and Home Soil (the Next Generation). I always enjoy the latter episode's depiction of the crystalline intelligence they encounter, upon seeing humans, saying through the Universal Translator, "Ugly! Very ugly bags of mostly water!"

Who exactly is doing this dismissing? When you ask why something is happening and that thing isn't actually happening, it is hard to answer without buying into the fallacy. All one can otherwise do is point the fallacy out. I don't know what scientists you have been reading or hearing, but the ones i read and hear do not behave as you say.

g

It could well be that there are other basic life forms out there. We only have one sample of life after all and one is not a very big sample.

Having said that however, there are many different environments both here on earth and on the few planets and moons close enough to us to have been explored to some degree. And so far we have only found carbon and water based life, which is a fairly strong indicator that that is likely to be the only type. Especially since, it is a reasonable assumption that a fairly rich chemistry is needed for the advent of life, and water is such a good solvent that we do not know of any other naturally solvent likely to produce anything like such a rich chemistry.

We do not dismiss the possibility of life based on other elements. We place emphasis on something we already know and largely understand. There are good reasons for that.

"most scientists" are not prone to speculate publicly -- no surprise.