You’re missing the use-cases of other systems. The design allows for multiple preferred languages because the implementer community has indicated that’s what they want. You are free to only expose one preferred language if that’s what your system supports, but you can expect to sometimes receive more than one from other systems. Typically in that situation, you’d just pick one as your ‘most preferred’ and ignore the others or require support for an additional extension that designated ‘most preferred’, putting the problem on the source system. In the worst case, you could treat multiple preferred languages as an error, but that wouldn’t be terribly interoperable/friendly.
All data structures in FHIR represent a compromise that may not always be optimal for a given system, but should be mapable to by most if not all systems.
Normally in this case I’d suggest submitting a change request if you feel strongly (using the ‘propose a change’ link). However, in this case, Patient is already normative. That means that breaking changes aren’t allowed anymore.