One reason is that a multisig address stands out and therefore might not be best for people who prioritize privacy.
Taproot has meant to have changed that, but I haven't seen multisig implemented until very recently Bitbox + Liana.
A strong passphrase is like a 2/2 multisig and it blends in well with single sig addresses. It's also good for creating decoy wallets in case of the so called $5 wrench attack.