I was literally thinking of that!

It makes a lot of sense to me. If you are just a low level schmuck of course you might need a magic sword or some sort of specific macguffin to bypass immunities. But once you hit level 10 you are reaching superhuman level. You've likely killed hordes of undead already. Might as well say you mastered the technique of the spirit tailor cut to deal with those spooky-wooky bedsheet ghosts. Unless you are fighting higher beings, trifling things like incorporeality should be beneath your concern.

D&D is not like Call of Cthulhu where you are facing things you have no hope to even comprehend, it's to go from a feeble villager to a mighty hero of legend!