As we know Private keys are generated only Once on every hardware wallet while setting up.
This is not exactly true, because you can generate new seed words as many times as you want if you reset your hardware wallet.You can even import keys that you generated yourself, that can be much more secure and it won't be connected with flaws or random generation by devices.
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?
Seed word generation in hardware wallets can be a issue but you are never exposing them to internet or broadcasting back to developers or anyone else (unless you received hacked malicious device.)You don't have to trust anyone and you can use dices or cards to generate your own BIP39 words and them import that in your hardware wallet.
Each hardware wallet is doing entropy in different way and you can read more about that in one of my topics:
https://bitcointalksearch.org/topic/seed-generation-in-hardware-wallets-5317199
There maybe possibilities of having loopholes while connecting to software's how can we trust it if they are created under any force?
Nobody is forcing you to buy those devices or to trust anyone, but you can always choose open source hardware wallet projects that have been tested by security experts.If you know the code you can always check everything yourself, but let's be realistic hardware wallets are no different from any other devices you use everyday like stupidphones for example.
