Your premise is false. Scientists don't only look at life similar to earth. However, they put most of the focus on that. They do it for a few very good reasons, for one, they know earth like conditions can produce life, therefore if they find earth like conditions they will likely find life. Secondly, given that we have a good example of life here, we know what to look for. What do you think would be the characteristics of silicone based life for example? Because we dont either, hence, harder to look for