Splitting XP isn't a solution for balancing more party members, as XP is a very small component in the balancing act. You have to account for a lot of things, like spells interactions, enemy power, placement and number, loot quantity and quality, and combat duration. There really are no easy way to allow for an arbitrary amount of party members and keep the game enjoyable for all settings.
It's definitely a solution, just not necessarily a perfect one. It's easy to do, simple to understand, doesn't affect 4-person parties, and will have a significant effect on party strength as 5e character strength is strongly tied to level. There's a reason 5e encounter-building rules, terrible as they are, suggest increasing/decreasing encounter strength depending on the # of PCs.
Also, there does exist some XP equation that is perfect for all practical purposes. It will be more complicated than simply dividing XP by # of participating characters, but not impossible to figure out. For a rough guess, Larian could load up one of their late-game encounters that they've balanced for 4 level 11 characters, add 2 PCs to the party, and do a couple tests at different PC levels to see what level produces an equally difficult encounter. Then simply scale XP such that a party of 6 reaches that level at that point in the game.