There are 8 schools of magic for a wizard to pick from at character creation.
There are not 8 schools of magic worth of spells for a wizard to pick from.
Why is this OK?
If a brand new player sits down and starts building a character and reads the 8 schools of magic, thinks divination sounds cool and loved the character Alex Verus who is a really awesome combat diviner and picks that as their school of magic... well they'd realize they'd been snookered because there is a shameful lack of divination spells.
Some schools are more equal than others, and shamefully so.
4x8 = 32. That is a very similar number to the number of spells per level for the first 3x levels.
3x8 = 24. That is a very similar number to the number of mid-level spells per level.
Is there any good reason why a 1st level diviner (or wizard from any other school) doesn't have a decent choice of spells that could be used to fill their damage/support/utility needs?
How many people over the decades have wanted to play a diviner... only to 'nope' out after looking at the diviner spell list?
Even more populated spell lists are not internally rounded with a good ratio of damage, support/control, and utility choices.
Yes, a wizard can go out of school, but in terms of building a character, how is needing to go out of school to get even the most basic of basics done a strong platform to feel like their school selection was a meaningful choice of their magic style and not just a bid to get an extra cast of a cherry picked power spell?
Especially if each school had 1 uncommon spell per level, and picking a school granted default access to uncommon spells of that school. That would really mix up building a wizard and provide the GM with reward options for the wizard to seek out.