It has to do with the time consumption.
Sure, you don't want to wait for 10-15 minutes? Wait a few seconds to do a few thousands hashes. You can't seriously trade security for a few seconds.
And considering that brainwallets that I've seen are browser based they are even more limited to the amount of computing power they can use so the process could be even slower than it could be.
Browser-based wallets are generally 🚩🚩🚩
Some tried to replace SHA256 with stronger algorithms such as scrypt but such projects don't gain any popularity since the idea of brainwallet itself is flawed.
It's outdated currently that we have standards such as BIP39, but it might wasn't a bad idea to create a secure brainwallet if the other choice was to keep a wallet.dat secure.
We may imagine that you want to launch hash function 20220807 times - and even if you forget that number but you remember period of time when the wallet was created, there is still way to restore it.
If you remember the passphrase, there's no problem. Just hash the hash, until you find the proper hash.