> I really don't understand why Apple is apparently incapable of making these laptops use standard, replaceable m.2 NVMe SSDs.

Coz there's no standard M.2 form factor NVMe SSDs without a controller chip, and even if you can find one, there's no standard protocol to talk to it.

Recall that 3rd-party drive manufacturers are really bad at properly making Self-Encrypting Drives (lots of HN stories previously, just search for SED). It's very likely a factor for Apple to decide to implement its own SSD controller inside the T1/T2 chip on recent Intel Macs, and directly on die on Apple Silicon, so that Apple can fully control the data written to raw flash and be confident in its own implementation of encryption.

Charging a premium for more capacity is a standard Apple practice anyway, and the fact that such security-focused approach makes it unavoidable is a side-effect.