Why are S-parameters are used at high frequencies?

The reason that we use S-parameters at high frequencies is because the S-matrix allows engineers to accurately describe the behavior of complicated networks more easily. If you know the S-parameters of a network, you can accurately predict its response to signals on any of its inputs. Some network analyzers, such as the Rohde & Schwarz ZNLE Vector Network Analyzer, or VNA, simply use S-parameter measurements. The R&S ZNLE is a two-port vector network analyzer with a full S-parameter test set for bidirectional measurements on passive components.