From my reading of the D&D 5e PHB, as well as other answers on this site (see Are melee combatants limited to standing around saying "I attack"?), it appears that martial combat is designed to meet these criteria:
- All martial maneuvers can be resolved using the same lightweight core mechanics (attack bonus, weapon damage, [dis]advantage, etc.).
- The only restrictions on what maneuvers can be attempted are the player's imagination and the DM's interpretation of what is physically possible.
Doing the same analysis for spellcasting, it appears that:
- All spells can be resolved using the same lightweight core mechanics (concentration, components, area-of-effect, saves, etc.)
- ...but many spells additionally define their own unique rules (see Does Burning Hands really require touching thumbs?).
- The list of available spells is complete. Freedom to improvise new spells is not granted to players.
So, if it was a conscious design decision to move away from the fixed power-list approach of 4e for martial combat to an improvisational model for 5e, why would spellcasting in 5e be limited to a pre-defined list of unique maneuvers?