My flip answer is simple: because there is no such thing.
Specifically, a checkride for a license indicates a level of privileges (private pilot, instrument, commercial, ATP, etc.), which specifies what you can do as a holder of that license. That license comes with restrictions specifying in which categories/classes (and types, for aircraft that require a type rating) those privileges may be exercised.
So a "Multi-engine Commercial Checkride" is really just shorthand for "A commercial checkride which applies to (possibly among others) multi-engine aircraft."
This is a somewhat academic distinction, of course, but there are two strong pragmatic reasons not to offer this as well:
- The number of such possible checkrides would be huge. Specifically, it would the [the number of possible ratings] x [the number of possible category/class/(type) to which it could apply]. At the moment, that's over 200 possible checkrides, and that's not even counting type-specific checkrides.
- It's pointless to log it anyhow, because all of the information is already present in the aircraft used for the flight. If you did your commercial checkride in a multi-engine aircraft, it's pretty clear that it was a "commercial multi-engine checkride." If you do your instrument checkride in a helicopter, it's obviously your instrument-rotorcraft checkride.