Bitcoin Core does not support BIP39, or any other seed word standard. You are correct that this implies backing up
wallet.dat and keeping it safe directly. This is a good idea in any case, even if there were a seed phrase, as the seed only helps recover the keys, but not information such as labels you may have assigned to transactions, or unconfirmed transactions.
Many other wallets do support BIP39 or other seed phrase standards. Some of these are very popular. I can’t tell you why common advice seems to ignore wallets like Bitcoin Core that don’t have these. Presumably, in terms of actual non-professional usage, such wallets are a minority.
The primary reason for Bitcoin Core’s wallet not supporting seed phrases is simplify because it predates such standards, evolves slowly (by being conservative about many changes), and until recently, introducing a feature for recovery from a seed would have been very complicated. This may well change at some point in the future.
wallet.dat file does use a BIP32 seed to generate keys from, but this seed is not something that takes the form of a phrase; it is simply 32 random bytes that are stored in the wallet file (in encrypted form if the wallet is password encrypted).