Your device will not attest to this if you install your own operating system, if you root your phone, if you do anything that they don't like, anything at all.
You install your bank's app and try to use it. The bank's servers ask for the attestation. You will not have one. They decide you cannot be trusted and deny you service.
Even if you can program your own keys into your device, nobody is gonna trust those keys. Why would your bank trust your own keys? They'll trust Google's keys, Apple's keys, the government's keys. You? You don't get to participate.
The corporations and governments want to own your computer. They demand cryptographic proof that your device is owned by them and that they have complete control. If you don't provide it, you're banned and ostracized from everything.