IMO-the annuity is used as a tool to illustrate the value of our A plan.
The way I think it works...is kinda like Social Security. There's a huge pot of $$ set aside as Pension funding. Each retiree pulls his or her share out of the pot. If the Investment returns are greater than the Company planned, well then they don't need to add a whole lot of $$ to the pot to keep the funding adequate (as outlined by Govt rules)
Pilots longevity is estimated, some will die sooner than the guestimate, some will die later. All of which has an impact on the huge pot of $$
Company could certainly Buy an Individual Annuity and meet their obligations. But if they do that, they've transferred the longevity risk\reward to a different party
But if you consider the PBGC, well, pretty sure they don't cover annuities.