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My reason been that you need to be knowledgeable and careful setting this up and any tiny bit of mistake might leave a back door or even make the airgapped device not longer airgap again, and sincerely newbies can easily make this mistakes and exposes the wallet.
That. Thats the main issue right there.
OP is advocating for a solution that demands a level of technical expertise most people simply dont have, and frankly, dont need. The whole point of a hardware wallet is to provide that secure, isolated environment without requiring the user to become an IT security expert. Otherwise, you are at a great security risk purely from the point of your ignorance or lack of expertise. Thats a gamble most users wouldnt, or shouldnt, take.
Most safe custody providers use this tech that’s why it’s called ‘cold’ storage. These company won’t reveal the wallet they are using because of security. Experts would suggest wallet to be custom built.