Why don't multiengine aircraft have to demonstrate spin recoverability?
Because it would impose severe constraints on their design.

Factors such as mass distribution make many twin-engine and multi-engine aircraft inherently resistant to spin recovery, despite the large rudder or large rudder moment-arm that is often present on these aircraft.  In many cases these aircraft are prone to flat spins.  Also, during the recovery from a nose-down spin, a large aircraft may exceed Vne before pulling out of the resulting dive, or may be overstressed during the pull-out.  In the real world, a pilot should avoid spinning a typical twin-engine or multi-engine aircraft, because recovery might not be possible.

"No multiengine airplane is approved for spins, and their spin recovery characteristics are generally very poor."
-- from https://www.flightliteracy.com/stalls-in-multiengine-aircraft/

Related questions on ASE:

Is spin recovery possible in an airliner?

Why aren&#39;t airliners spin-tested?

Other related links:

https://www.flightliteracy.com/stalls-in-multiengine-aircraft/

https://www.pprune.org/flight-testing/226351-spinning-airliner.html

https://www.airlinepilotforums.com/hangar-talk/6370-spinning-airliner.html

google "spin recovery twin" or ("multi") for much more