Totally just guessing: Maybe to reduce inbound spam that's forwarded to the buyers? If it was one central email address, spammers could just send a ton of images/PDFs to all known prefixes of that central domain, but needing to guess the prefix + right domain adds a layer of indirection? They may not do much validation on the forwarded tickets, just store it for disputes
I was curious the last time I sold tickets so actually called them asked about this. The emails are aliases to verify the tickets are valid, then they are forwarded to the buyer.
Usually the format is <email prefix of buyer>@<stubhub controlled domain> and the tickets are forwarded pretty quickly. This way stubhub can actually validate you sent over a PDF + the contents of those PDFs or images if the buyer disputes delivery.
I have point type A and I want type B - what's the most economical path to walk between the two point types.