It's that complicated in part because US-based airlines (for historical reasons) insist on calling the marginal domestic improvement over economy "first class", even though the rest of the world consistently calls the the next class above economy "business class". (US airlines only call it "business class" on 3-class planes domestically, but bow to the worldwide usage on intercontinental flights where they compete with airlines that always use the worldwide usage.)

The other reason it's complicated is because there is no standard for fare codes worldwide. And as long as they stay one letter long, there probably never can be, because there are needs unique to each carrier, and thus each carrier tends to use up most of the alphabet but in different ways. To have standardized fare codes worldwide, you'd have to "reserve" huge portions of the alphabet, thus not leaving enough fare codes left over for things unique to an airline (like "premium economy" seating on airlines that have it or more special types of awards on airlines that have that instead).

And since allinaces and partnerships are eternally in flux, it's no more possible for all partnered airlines to standardize (without the whole industry standardizing). For example, Alaska partners with both AA and NWA and several other US airlines, each of which are in a different alliance; no way thus could Alaska have complatible fare code letters with all of their partners unless all the alliances were compatible with each other!