Why don't microwave oven transformers state their output voltage?
Microwave ovens are made in large numbers by many manufacturers. Each manufacturer has a specific circuit that requires a transformer with very specific requirements. These requirements will be laid out in a written specification that the manufacturer's vendors will bid on and the winning vendor will then produce that transformer to that specification. The transformer is intended only for use in microwave ovens.  It is not intended to be sold separately or to be re-used after the oven has completed its lifetime.  Thus there is no need for the transformer to have any markings indicating its operation. This is true of many components used in both commercial and military applications. It does  it harder for experimenters to salvage parts but that is the way it is.