But it's not the case 'they were just there'. They made a conscious decision to leave the FL and form the PL. It's not something that just happened; they could have voted against it.
No, it's because Sheff Utd were founder members. They held a meeting and signed a document, the Founder Members Agreement, setting out the way the PL would be run. It wasn't because they happened to be there, they had an active role in setting it up