The reason is obvious, which is why I didn’t think I had to say it lol
Imagine if you went to work at a bank in IT for example. They issue you a laptop. You go “oh no it’s ok I’ll use my own, I assure you it’s secure and safe”.
You connect to their network and instantly their network is compromised, every device on the whole network gets a cryptolocker virus.
You said it was secure though, so how did this happen?
You can trust it all you want, but other systems absolutely do not have to, and anything that requires security assurances will not and should not trust it. The device owners word/assurance means nothing.