Because in science we tend to use "Occam's razor" approach - we don't make assumptions that are more than necessary to explain the behaviour of something. Most

Because in science we tend to use "Occam's razor" approach - we don't make assumptions that are more than necessary to explain the behaviour of something. Most adherents of the anthropic principle assumes "Oh hey, we probably live in a multiverse" to justify it, but we have no strong evidence that that is true. So making that assumption is difficult to justify. It may be true! But we don't know it is, and until it can be proved, the theory rests on shaky ground. It might be that another, simpler theory can explain the same behaviour. Should we assume that a simpler theory is true? No, not necessarily. But it lies on the adherents of a more complex theory to come up with reasons why that theory is right, by making predictions that can be tested.