Mutating a std::vector is much more difficult because you need a coarse lock and serialize every access with it - there's no other option. Linked lists, on the other hand, can be made thread-safe with a single coarse lock, or many finer-grained locks, or atomics, or fancier tricks like RCU.
All of that is difficult, sure, but what is less difficult than thread-safe linked lists?
Mutating a std::vector is much more difficult because you need a coarse lock and serialize every access with it - there's no other option. Linked lists, on the other hand, can be made thread-safe with a single coarse lock, or many finer-grained locks, or atomics, or fancier tricks like RCU.
All of that is difficult, sure, but what is less difficult than thread-safe linked lists?