They’re a business. Reducing their costs (while charging you a premium) is absolutely what they do.

Apple’s whole deal for decades now has been building a vertical supply chain. Using their own SSD controller is one less component they have to pay others for.

They just don’t give a shit about downsides: aftermarket repairers or user upgradeability.

It’s more cost effective to integrate the controller.

Being worse for customers is just a happy accident.

It’s replaceable, it’s not upgradable.

Apple doesn’t use standard NVMe M.2 drives. The controller is built into the SoC rather than being on the storage device itself.