IANABanker but I suspect it was appealing for the 4 "real" banks because they barely had any work to do. From their perspective they had one single corporate customer called "Synapse" with an account that had a nice huge balance, and life was easy.

Contrast that to if the same sum was divided across thousands of individual accounts. Each would involve a certain amount of regulatory reporting, identity proof, pestering people to pick a beneficiary, monthly statements, etc. In addition there would need to be some way for Synapse to do deposits/withdrawals on the owner's behalf, and more of the sum would be FDIC insured meaning more money would be going out to the FDIC in insurance premiums.