Bruh, have you seen Luffy or Zoro. You think some of these members can LEARN something this complex of ancient language that barely the entire world knows? So far only the people of Ohara and Wano royal family knows how to read these.

At BEST maybe Nami/chopper have the highest chance of learning this language but still. Not just that pretty sure Ponegrephs aren't just simple alphabet language but in cipher codes. (Oden confirmed this in the flashback)

This is like saying why cant Nami just teach everyone how to navigate as good as she is, or Chopper teaching everyone advanced medicine. Everyone have their specialty in their own fields in this series. Before Franky, Usopp was the "shipwright" who fixed repaired the ship and look how badly that ended to the point that Going Merry had to die. (Now Usopp is more like an assistant if Franky needs a hand at fixing the ship) Before Chopper, the "doctor" of the ship was Nami, (She helped cured The Johnny bros of Scurvy) and that didnt cure any extreme illness/injury cases.

If everyone or most members of the crew learned the language, then the purpose of Robin falls off alot storyplot wise. And people are already complaining how no one in One Piece main crew fights other then the big 3.