Because you’re white-rooming the supposed issue into something it isn’t.
It literally just breaks down the process of choosing spells into level by level chunks, rather than a no-guidance pot, which is damn near objectively easier for the vast majority of people.
Especially because the number for each level is literally already there. It’s the same as your number of spell slots.
The new way is much easier, especially on level up. Previously on level up you got one more spell to prepare or if you are a half caster maybe one or maybe zero, unless you also got an ASI that level and then it was 2 .... or 1 ..... but then if it is an odd level you have higher level slots so you probably want to drip a lower level spell for a new higher level spell ..... so you got 5th level and you got 1 more spell to prepare, but you also want to drop magic missile, so that is 2 more third level spells to prepare now ..... oh and you got two more spells but you got those from feats so they don't count and this one is a prepared Wizard spell but this other one is a known Bard spell ..... and next week when you come back after a week off - wait I always have magic missile prepared, I must have screwed this up ....
I would say at least 20% of the time when as DM I looked at a mid-level casters sheet we found an error, sometimes we evem "found an error"
that wasn't really an error.
Of course that never, ever happened to me as a player ... and if it ever happened as a player I never would have quietly fixed it on my sheet without admitting it!