The linear time behavior of cuckoo hashing is not triggered only when the table has to be enlarged, like standard containers (say, std::vector or python list): a linear numbers of keys can be "cuckooed" out of their slots for an insertion even when the load factor is under the capacity threshold.

In standard containers if you pre-define the capacity (say with std::vector::reserve) all the insertions are guaranteed to be constant time. Cuckoo hashing can't give this guarantee.