But, there's the rub. See, 5e doesn't actually use 1-2-1 counting. So, you can move diagonally 6 squares on a grid. But, for some bizarre reason, spell effects still use the 1-2-1 counting. Sort of. It's actually the worst of all worlds.
In any case, it's easy to explain why fireballs are square on a grid - ease of play and a gridded map is not, and never has been, a perfect representation, but, rather an abstraction. So, we have square fireballs, because it's a heck of a lot more convenient to play.
edit to add -
Hang on. Neat pixelated cubes? Sorry, that's 3e, not 4e.