Why is your grandpa or fictional billions users being MITM’d?
Passkeys are an end run on general purpose hardware largely under your own control, which is a much bigger issue in scale than dealing with customers who can’t access the services they pay for because of some byzantine security or know your customer laws/procedures.
For their bank accounts, search tech support scam on youtube to see how that goes. Btw passkeys do nothing to stop this, they probably make it easier since grandpa doesn't have to search for a password or unlock a password manager to send his money to the scammers.
Why are they being MITM'd? Because some people want money and don't mind breaking laws.
There is a reason that banks send increasingly strident text messages that they will never ask for this code.