Mainly because the auto companies are already on the verge of bankruptcy, and putting an additional burden on them both pushes them further to the brink, and makes it more difficult for the unions to extract concessions from them.
Besides, they don't benefit if GM puts money into the pensions anyway. The PBGC will make up the shortfall with taxpayer money if GM's pension funding is inadequate.
Yes and no. PBGC will not pay the full pension amount. Workers take a big reduciton. Except the bigwigs, of course, whose full pension amount is guaranteed.