The MCB et al spent a TREMENDOUS amount of time and effort to achieve adequate couplers. Then the USRA did a TREMENDOUS amount of work on draft gear. That work continued into the 60s [and still does] to 'perfect" long travel gear to reduce shocks.
Tank cars, and passenger cars, to name two, don't like separation- it leads to telescoping and rollover.
As in EVERYTHING ELSE, except maybe, and that's maybe, for space travel, absolute is unachievable
There is no justification for an indestructible knuckle, nor good reason to make it an intentional weak link. Start your homework for couplers around, say 1885. In the ensuing years hundreds of couplers were abused to failure- MCB standards even included a special testing machine etc.
O yes, a knuckle is heavy, more than once I've played trainmaster to a crew who had to walk a replacement back 50 cars- they were grateful.
I once had a complete coupler, ripped off a tank car in a derailment, in my dorm room, on the second floor. The Special Agent suggested I put it on the RR's local scrap pile.