Pis ressort doesn’t make sense in French.
Anyway, the reason why aller doesn’t mean ressort is because pis doesn’t mean dernier.
Pis is the neutral form of pire, maybe the only word that survived the neutral case disappearance in French, and means "worse".
Aller is the substantivated form of the verb aller (to go), so pis-aller literally means something like "worse way, worse solution" but is mostly used nowadays to mean an unsatisfactory workaround, something doomed.
Let’s try to make it clear: "pis-aller" precisely means "to go with the worse choice". It’s basically a poor workaround, doing what we can with what we have.
"Dernier ressort" is literally "the last chance, the last way out". "Une solution de dernier ressort" is somewhat worse than a "pis-aller": it’s "the very last chance" vs "not the preferred or best choice".