"Building blocks of life" is a phrase with no strong definition. Science journalists like to apply it to anything from water to DNA chains, though those are very different molecules in terms of complexity.
0: https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment
Those may seem complicated, but you have to also remember an "amino acid" is the entire family, and carbon tends towards "a bit complicated" naturally because it has so many bonds it needs to fill in. Fill an environment with carbon and the right other elements and it isn't "surprising" that amino acids form, it's inevitable. Like, literally unavoidable, you couldn't stop it if you tried. "The right environment for amino acids" would also be full of many other larger structures. It's a thing carbon does no matter where it is, not an exception. They would just be, if you'll pardon the convenient anthropomorphization, pointlessly larger structures, not doing anything in particular or having any particularly interesting properties beyond size. You get similarly large carbon structures in the soot of a fire, for instance.
Compare with hemoglobin, just to pick a popular protein of moderate size: https://en.wikipedia.org/wiki/Hemoglobin#Structure_of_heme This is a specific combination of 574 amino acids, in the correct chirality. Some deviations are possible that will retain functionality, it isn't that exact 574 amino acids, but there's limited variation possible. "Things useful for life" are much more than just bare amino acids.
This isn't something that should be downplayed; it is legitimately interesting, but at the same time, it doesn't materially move our probabilities on any particular theory of life because it was always predictable that amino acids would exist elsewhere in the universe. At a bare minimum for excitement would be demonstrating some sort of significant chirality preference. That would turn some heads.
So it is a data point supporting the idea that they could have been a source of these molecules. You may or may not find that exciting.
It would actually be more surprising if it turned out the molecules we are built from do not arise easily and naturally.
This doesn't mean that all life would be based off them though.
>how is it not possible at least to say that we're not the only place in the universe where life exists?
Almost every expert believes that. In fact, it's entirely possible life didn't originate on earth in the first place. Not that it necessarily has an origin outside the solar system, but who knows.
