I agree with ranochigo. If done perfectly, then yes, a seed generated using /dev/urandom or fair and random coin flips on a clean airgapped and encrypted device is going to be just as secure as a seed phrase generated on a hardware wallet (perhaps even more so if your hardware wallet is not fully open source). The issue is the level of complexity in doing that. Almost everyone can plug in a hardware wallet, follow the easily laid out instructions, and generate a seed phrase securely, whereas even fairly tech savvy people can mess up when trying to create an airgapped device and generate a seed phrase themselves. If you don't trust the seed phrase that the hardware wallet has generated for you, then why are you trusting the hardware wallet at all? If you want to generate your own seed phrase, then you might as well just set up an Electrum cold wallet or similar.

I use both hardware wallets and airgapped and paper wallets with seed phrases I have generated myself, but I spent a long time testing my set up to be sure I was happy with the security of the seed phrases I was generating.