Even though I don't necessarily like it, I understand why they solder the RAM on the SOC: Higher bandwidth/greater performance, better power efficiency, etc. But they have no excuse for the SSD.
Why does the controller need to be on the SoC when the market already has SSDs with built-in controllers that follow a standard and allow you to easily replace it?
It doesn’t have to be. It is an engineering decision that has design consequences. Integrating it likely has advantages in packaging, performance, power consumption, etc.
The shortage that connects to a modern Mac isn’t an SSD — it’s raw NAND.