A b-tree grows and shrinks gracefully because each update modifies O(log n) pages. And the base of the logarithm is pretty big.

A typical hash table is not graceful in this way, because while each insert is O(1), you occasionally need to rehash everything into a bigger table, and that is O(n). As I noted above, linear and extensible hashing avoid this problem. An insert that forces a reorg just reorgs one bucket.