a) Your username is your email address. While I, and probably a few others have plenty of email addresses to choose from, I guess most owners have one or maybe two.
We also know that people are people, and they regularly use the same password for many services. So with a bit of trial and error, and with a few of the leaked password-lists that are readily available on the net, it is quite probable that you would manage to get hold of at least a few combinations that are valid for logging in to Teslas services.
And as far as I know, there is nothing that stops you from being logged in from multiple devices at the same time. This means that you would neither know if someone else is logged in as you right now.
Nothing stops me from collecting usernames and passwords for a while, and simply checking if any of the cars owned by these users are somewhere in the neighbourhood, parked, waiting for me to pick them up.
I don't need to steal your phone, I can just as easily log in using your credidentials from my own phone.
I don't say this is a big risk, cars are stolen all the time, even without this possibility. But at least you should make sure you use a unique and really hard to guess password for your Tesla login.
I do. You might do it. But everyone does not, and that is a big risk.