There are only so many FedEx jobs\seniority numbers.

The implied intention here is to come to FedEx for a year or two until the OP can return to his mainline job.
The why should I have to resign just to be hired at FedEx mindset.

No one truly owes any obligation to another pilot seeking employment, but, IMO-it's an interesting display of a lack of integrity.

There's a whole slew of pilots hoping for just One big airline Seniority number.

Definitely not an easy situation, but I sure didn't get the impression the OP was just biding his time at his mainline gig before being able to switch over to FedEx