Probably because that was the case under JARs. JAR-FCL 1.180 stated "....the privileges of a holder of a multi-engine IR(A) are to pilot multi-engine and single-engine aeroplanes under IFR....". However, that privilege was not carried over to Part-FCL and, unless and until the law is changed or the competent authority issues a derogation under Article 14(7), cross-crediting does not apply to the skill test for initial issue of an IR.
Another example of the dangers of allowing a bunch of inept bureaucrats loose on something they patently don't understand.