From the tag info (highlight mine):
Electrical Machines are Electro-Mechanical energy converters. They maybe classified into motors (electrical -> mechanical), generators (mechanical -> electrical) and transformers (electrical -> electrical).
The inclusion of "resting" magneto-electric devices in that category is imho controversial, but that's not the point:
Since transformers are significantly different from motors and generators, a tag covering all three feels less helpful:
Whilst you cannot, without knowing the direction of power flow, tell whether a specific machine is a generator or a motor at any point in time, so it's helpful to have a general tag for both.
A transformer is a transformer, and no (intended) mode of operation changes that¹.
Since someone asking about a transformer is likely to use that tag, shouldn't we exclude that usage from the electric-machine tag description?
¹ could of course also be an electrical bonfire.