"Couldn't just", might be, probably not. Face-ID is a pretty complex and very highly integrated system. The dot pattern can't be changed, because each dot in the pattern (~100 dots or so) is actually a VCSEL laser. The large constellation (>30k dots) is created by a diffractive beamsplitter. The sensor is probably custom, so I'd wager the CMOS IR sensor is actually physically the thing that's paired to the Secure Enclave. I doubt there's just an unencrypted MIPI link running from some random 1/6" OmniVision sensor to the CPU.