I'm guessing that the PHB design team wanted to give enough margin that a player with a sorceror PC could have an interesting time trying various metamagics on various spells, and find some good combos; while not putting, directly into the PHB, anything which would make the game stop being fun for all the other players. Since the PHB is written with the goal that even a first-time DM and first-time player can use it, they had to be cautious about that margin.
My goal would not be "design the classes so that a 5th level Wizard, 5th level Warlock and 5th level Sorceror are equal in power". My goal would be "design the classes so that a 5th level Wizard, 5th level Warlock and 5th level Sorceror are all fun to play, even if there's one of each in a PC party".
If one PC is outshining the other across the board... then the DM can figure out a way to address that. I would much rather trust DMs to adjust and house-rule as needed, than build constraints into the PHB which prevent comparison issues from ever arising at any table. If a first-time TRPGer is playing a Wizard, and a player with two decades of practice at min-maxing is playing a Sorceror, there's one set of balance issues; if you have those same two players at a table and the newbie plays Sor and the min-maxing veteran plays Wiz, then *they will still have balance issues*, no matter what's written in the class descriptions in the PHB.
My goal would not be "design the classes so that a 5th level Wizard, 5th level Warlock and 5th level Sorceror are equal in power". My goal would be "design the classes so that a 5th level Wizard, 5th level Warlock and 5th level Sorceror are all fun to play, even if there's one of each in a PC party".
If one PC is outshining the other across the board... then the DM can figure out a way to address that. I would much rather trust DMs to adjust and house-rule as needed, than build constraints into the PHB which prevent comparison issues from ever arising at any table. If a first-time TRPGer is playing a Wizard, and a player with two decades of practice at min-maxing is playing a Sorceror, there's one set of balance issues; if you have those same two players at a table and the newbie plays Sor and the min-maxing veteran plays Wiz, then *they will still have balance issues*, no matter what's written in the class descriptions in the PHB.