A century ago or so, it was popularly thought that Mars was covered with irrigation canals, obviously created by some advanced form of life. That was debunked and the hope shifted to maybe there being some fungus or plants... Such were not found so it shifted to microscopic bacteria. There is perhaps some evidence of those, but nothing conclusive, so now people are talking about cracking open rocks to find the fossils of bacteria...

This all has an uncanny resemblance to the "god of the gaps" phenomenon. As science explains more of our world, the religious cram their gods into smaller and smaller gaps in our scientific understanding. And as science shines light on the apparent sterility of Mars, alien life enthusiasts cram their proposed aliens into the ever smaller gaps in our knowledge of Mars.

Oh, life being common is an older idea than that. In the 1700s it was common belief that each of the other planets in our Solar System was inhabited, because otherwise their existence would be a waste, and because of vulgar Copernicanism.

People can expect that life is common any amount they like (well, if they stay within the 1 in a few thousand planets upper bound we have as a fact nowadays).

My problem is with people insisting that's a certainty.

I'm pointing out that common belief was hilariously wrong in the past, so there's no reason to think it's not hilariously wrong now. The meme that "there's lots and lots of stars, so life must be out there" is not sound reasoning.