Apple uses their own T2 chip as the SSD controller, and it’s integrally tied to Apple’s new file system.
An M.2 drive has its own integrated controller onboard, and cannot interface (and is therefore not compatible) with Apple’s T2. The T2 expects to see raw NAND chips, not an additional SSD controller behind a PCIe 3.0 interface.
Soldered chips are more reliable, and the T2 more secure, than M.2. Both benefit the customer.
There is nothing the M.2 form factor connector can't do that the Apple proprietary solution can and it's nonsense to accused about the reliability which is patently false.