It's because the phone is a two-factor token that everyone has with them. With a secure processor being the hardware token and fingerprints or face scans biometrics. This makes it ideal for saving such sensitive data. I most frequently use it to digitally sign documents in a legally enforceable way.
The card you linked is similar, and a smart card was one of the previous versions of our system. The goal here was to make it universally accessible, and a smartphone is perfect for that.
I have two of them, from Mr. Robot, for decoration - just like I have other statues for the same purpose. I took them out of the box though, I don't get why anyone would do that.
Have fun exploring! I just have a simple Raspberry Pi at home with a few services, after working with this stuff all the time I rarely feel like tinkering at home :D
Well it has read all we know about it.