There are way too many variables to know how every Evoker is going to fight in any given battle, and having a variety of spells (via a full spell list) allows for more options, depending on just how and when the Evoker is encountered.
Why have magic missile? Because sometimes guaranteed damage a PC cannot save against or evade is better.
So, I can have a complete spell list with lots of options of how I want to run my Evoker, or I can have a fixed stat block with limited options...
And yeah, I know I can "give whatever I want to change a stat block", but I am not fond of doing that since it basically means I might as well just throw down whatever info I want and not worry about game systems, mechanics, balance, or a myriad of other things.