Why isn't the evidence $p(x) = 1$ if it's an observed variable?

“Every explanation of variational inference starts with the same basic premise: given an observed variable $x$ , and a latent variable $z$ , $$ p(z|x)=\frac{p(x,z)}{p(x)} $$ and then proceeds to expand $p(x)$ as an expectation over $z$ : $$ p(x) =…”