There a good reasons for organic chemistry being the main one. Carbon is abundant and can be combined in numerous ways to make complex molecules and stable ones at that. There are very few other elements in the periodic table that can hope to offer similar properties. So carbon based life is not just a guess, its a high probability.

I don't know where this myth comes from that scientists have some sort of strong belief that no other form of life is possible, but it's not true. What we have is a lot of ignorance, so many scientists will often concentrate only on those forms of life (plural) we know exist because we share a planet with them, because those are the forms of life we can actually discuss with confidence, but I doubt you can find a serious scientist who will confidently proclaim no alternatives are possible.

There are good reasons to believe that ours is quite likely and good reason to be skeptical of alternatives, in the proper scientific sense, but certainly not disproved or something.

> I doubt you can find a serious scientist who will confidently proclaim no alternatives are possible.

Undoubtedly. The problem is that very few people, even among the well-educated and technically proficient, actually form their views about the state of science by talking to scientists.