Why you step up or step down voltage why not current?

The current is automatically stepped up or down when the voltge is stepped down or up by a transformer.

Most Transformers are designed for a constant voltage, and the current varies depending on the load. Current transformers are also made to detect the current in a power cable so that it can be measured.

Another Answer

It's a misconception that transformers step current up or down (with the exception of current transformers, that is!). Transformers step voltages up or down. The secondary current is determined by the load. If there is no load, then there is no secondary current; if there is a heavy load, then there will be a heavy secondary current.

The secondary current then determines the primary current, because the primary power must match the secondary power (ignoring losses), so if the secondary current increases, so must the primary current. The current itself isn't stepped up or down.

<<>> Normal transformers work with constant-voltage supplies and are designed to step the voltage up or down, and the current is incidentally stepped down or up in the same ratio as the voltage.

Current transformers are designed to step current up or down (usually down), and are used most commonly to sample the current in power conductors.