You've never once flown an itinerary with two airlines on it? That's a very routine use case, for example hop on an American flight to London connecting to a BA

You've never once flown an itinerary with two airlines on it? That's a very routine use case, for example hop on an American flight to London connecting to a BA flight to Glasgow or something? In that case you bought your ticket from a different company (AA) than the airline you're flying (BA on the second leg). That's just one of a million examples. For complex travel arrangements it's just normal.