As we know Private keys are generated only Once on every hardware wallet while setting up.
We can't generate it many times or as we want,
Not true. The private key generation on every hardware wallet relies on SEED phrase (+optional password) and hierarchically deterministic path. One can generate them as many times as he/she wants via new SEED phrase which follows HW reset.
How do we trust anything or anyone that it's a complete random generation, we are not in it we are trusting manufacturer's? It's like trusting third party? If any force make them cheat on it how can we protect ourself because people are trusting hardware wallets blindly thinking its 100% safe many have lifetime saving's on it !!
Folks who are special paranoiacs can generate
SEED phrase from the scratch via dice usage and put it into airgap wallet like
Ellipal Titan. The latter has no channels to communicate with its manufacturer.