Neonchameleon
Legend
The problem here is that you are conflating in character and out of character decisions. For a wizard their spell list is largely an in character decision; it's what they have chosen to research. And, assuming that Fireball is the best third level combat spell (there is a good case for this) and that the character routinely goes into life threatening combat, the only reason for a wizard who can learn it to not take Fireball over e.g the more thematic but generally much useful Vampiric Touch is that they are someone so extra that they consider sticking to their theme more of a priority than lowering the risk to their life and the lives of their friends.If a person's Necromancer has the same spell load-out as any other caster... that's the player's decision to not stay on theme, not because it's a problem with the class. It's not WotC's job to force players to stay on theme by restricting or shortening spell lists.
If a player just "can't help themselves" and takes Fireball and always uses Fireball (even though they are a Necromancer) because it's the "best 3rd level spell"... then that's the player caring more about optimization than they do playing a character. That's not WotC's fault nor their responsibility to fix.
For a sorcerer of course their spells are what they get and are frequently not an in character decision. A sorcerer who picks Vampiric Touch over fireball is being played by a player who thinks that it is more of a priority for their character to stay in theme than it is to make the game easier. But they are not risking their own life.
It's not WotC's job to force players to stay on theme. But the theme of the wizard is "I can learn any arcane spells and can change them with little notice" and this directly undermines other thematic casting choices. This is why the wizard should be a subclass of sorcerer, and necromancer should also be a subclass of sorcerer.