Earlier quoted context omitted.
It makes perfect sense for a regulation that intends to protect the retail customer's interests. I suspect Robinhood would put customer money up for collateral if they were allowed to do so.
What is that responding to? The problem is the lack of a consistent threat model for whom you're protecting and what you're protecting them from . Half the time it's "the consumer could reneg on the purchase" and half the time it's "the shares could be stoken from the consumer". And 100% of the time, the super-confident, I-get-this-and-you-don't explainer doesn't realize the inconsistency.
I can't answer why customers with cash in hand couldn't directly settle the trade. Maybe most of the time it doesn't matter and it's easier for brokers to have one settlement process. Maybe the industry has come to depend on the delayed settlement in some unrelated way. Maybe it's something that hasn't fully been adopted due to change taking time. Maybe the referenced regulations get in the way of an uncollateralized process.