That's true, but we can make some educated guesses based on the general characteristics of life. In particular, the binding energy is a goldilocks problem (too much you can't recombine too little you don't stay together) that favors water and carbon. A good overview of the conventional wisdom on this is presented in this video "the aliens will not be silicon" by acollierastro, https://www.youtube.com/watch?v=2nbsFS_rfqM
