Huh?

Could you explain this because it's very confusing. Or are we not talking 5E?

It's a fireball.. that has a big AoE. So if it's a roughly hand sized sphere but it deals damage in a big sphere.. logically it has to explode, right?

Beyond that it's got the word "explosion" in the spell description, at least on dnd beyond.

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame.