> Why would you sign into Ticketmaster with an email address you don't have access to and use it to buy tickets?

Because in the context of signing in, its role is that of a user ID.

Ticketmaster spams that address constantly. It's a valid email address, to be sure, but they've trained me over the years never to look at it. They certainly didn't do any multi-factor authentication when I bought the tickets, only when I was preparing to use them (despite having accessed them on that very device two days earlier).