If I remember well, AES (then named Rijndael) won the competition because it was slightly faster to implement as one of its competitors, Serpent, which erred on the side of caution, using 32 rounds as where most probably, 16 rounds would have been sufficient. In a way, the competition was rewarding "highest security per clock cycle" (way of speaking). That was almost 2 decades ago, when computers were still much slower than today, so clock cycles were still expensive, and too heavy cryptographic burdens might make it hard to use on home computers. However, today the problems of encryption are rather the large amounts of brute force that one can apply. This is why one uses "inefficient" key derivation functions, to slow down brute force.
My question is: why doesn't this apply to block cyphers ? Wouldn't it be a good idea to have "inefficient" and slow block cyphers ? Wouldn't something like Serpent, but even with say, 4096 rounds, be preferable over an efficient algorithm like AES, so that it takes much more time to use in a brute force attack ?