A car's generic "check engine" light makes it easier for dishonest mechanics to take advantage of unknowing customers. Considering other car features that are federally-mandated — like tire pressure sensors and airbag warning lights — wouldn't adding one to actually help the consumer make sense? It may be the only type of federal-required feature that makes sense.
This is why we need a federal mandate that bans the generic "check engine" light in new cars and instead requires, on dash, OBD-II codes and a basic description. The only rational reasons it hasn't happened yet range from a best-case scenario of simple manufacturer desire to build as cheaply as possible, to an actual deliberate campaign of forced ignorance in order to keep dealer network profit streams.